The Education Secretary said it was crucial that all schools reopened as soon as possible in order to protect the welfare of children and allow them to progress.
He has hit back over claims from teaching unions that the decision to reopen schools has been rushed, saying he was confident that the right safety measures were in place for primaries to reopen this week.
The majority of primary schools across the region were today running staff training sessions before reopening to pupils from Reception, Year 1 and Year 6.
Many local authorities have left the final decision to governors and headteachers, while some have ordered schools to remain closed over fears that the coronavirus 'track and trace' system was not yet robust enough.
South Staffordshire MP Mr Williamson told the Star it was now "safe" for pupils to return – and also vowed to take action if schools refused to open without good reason.
"All schools need to start opening up," he said. "It is in the interests of the children's learning and wellbeing.
"Where there are practical issues in terms of reopening, I'm very happy to have a discussion about how we overcome them, but we have got to put the interests of children first.
"The safety of staff and pupils is at the heart of this decision and has guided us from the start. At this point there is no reason why schools should not be reopening.
"We accept this will be a gradual process and we have shown a lot of flexibility to enable schools to open up in a phased manner.
"The steps that have been taken, with much smaller class sizes and strict guidance have been brought in with everyone's safety in mind.
"We are all in this for one common aim, which is to make sure children receive the benefit of a good education. Quite simply, having schools closed does not benefit children."
Schools across the region have been closed for regular classes since the lockdown started in March, although many have remained open for vulnerable pupils and the children of key workers.
Mr Williamson said he was determined to reopen them for more pupils as soon as it was safe to do so, due to the "hugely detrimental" impact that long term closures would have on their progress.
The Government is understood to have followed medical advice when deciding on which year groups to bring back first, with experts believing that secondary pupils are likely to have higher levels of transmission of the virus due to them having wider friendship networks.
Secondaries have been told to reopen to small groups of Year 10 and 12 pupils from June 15, although some pupils may not return to school until September.
Mr Williamson said: "In a perfect world we would see schools returning exactly as they were, but we realise the challenges we have been dealing with with the virus, we have got to bring schools back in a cautious, phased manner.
"That's why we are taking these very small steps forward, and we'll be making an assessment on how this has gone before we start making decisions on bringing other year groups back.
"We have to be sure that the system works as well as possible and that schools are coping with the numbers of pupils they have coming in."
Mr Williamson said he had been concerned about the impact of school closures on the mental health of young people. While he was keen to get as many of them back into school as possible, he said, it was clear that any sort of return to a "normal" school day was still some way off.
"The physical and mental wellbeing of children is so important and getting back into school can only be for the good," he said.
"Any of us with children have all seen how much they are missing out.
"That's not just on classwork, but on the ability to socialise with their friends. Many of them haven't seen someone their own age for over two months now.
"We don't want to see this go on, so when we are in a position to bring back schools fully, we will.
"That is what we are working towards, but it is a long journey and we have to take it one careful step at a time."
The Education Secretary also said it was important not to view the return of schools in isolation, pointing out that it was part of a "wider plan" to ease lockdown measures.
Groups of up to six people from different households are able to meet outside from today, while outdoor markets and car showrooms have also reopened and will be followed by non-essential retailers from June 15.
Mr Williamson said: "This must be taken in a broader context, where in other areas of our lives we are starting to give people the opportunity to do more, whether it's families who are able to meet or garden centres and markets opening back up.
"These are all positive steps and the decision over schools was taken with these issues in mind. Safety is at the core of the decision to reopen schools, but we also realise that we have got to start moving the country forward."
No substitute for classroom-based lessons
The Education Secretary has hailed the success of remote learning during lockdown – but said it was no substitute for classroom-based lessons.
Many schools across the country have run virtual classes since they closed their doors in March, with pupils tuning in online and submitting work remotely.
Gavin Williamson said the system had proved to be a huge success and could potentially be used more often to supplement on site learning after the coronavirus crisis.
But he said that while there was "an enormous" hunger for online learning, it could never replace the benefit of having a teacher standing in front of a class.
Mr Williamson hailed the success of the Oak National Academy, the Government's online learning resource which delivered six million lessons in its first three weeks.
He said: "What we have seen is that there is a reality with remote learning – when it comes to online lessons and support – that it has been a success for schools which have used it.
"The success of the Oak National Academy shows the enormous hunger for this type of learning there is, and also how it can be used to supplement the work that schools are doing.
"However, while it has undoubtedly been beneficial, it is no substitute from children being in the classroom with their teacher and having the opportunity to have work properly explained to them and ask questions.
"Online learning has been a huge bonus during the lockdown and it will be with us for a long time, but we should never underestimate the value of having a teacher in front of a class.
"It is important that schools return to something like the pattern of work that they have always done."