今年の４月から来年の３月に生まれた子供は同学年になるって英語でなんて言うの？

"Children who are born between April and march of this year will all be in the same school year" This sentence is implying all the children born between the months of April and March this year will all be in the same school year at school, school year is another termination used for grades in school.

The years of school are referred to as 'grades' and there is a cut of from children of age depening on when their birthday is so you would say 'Children born between Aoril of this year to March next year will be in the same grade'

You want to explain that children born within the period beginning April 2019 and ending March 2020, will be in the same grade. This means all the children born in April 2019, and all those born every other month up to, and including March 2020, will be in the same grade. When you say 'every child', you mean 'all the children', born in the months beginning April this year or any other year, up to, and including, the month of March, the following year. So, you may tell someone as follows: Children born in the period beginning April 2019 and ending March 2020, will be in the same grade. or All the children born in the months beginning April this year and ending March next year will be in the same grade. or Every child born in the months beginning April in a particular year and ending March the following year, will be in the same grade.

You want to explain that in Japan, children born from April of this year to March of the next year will be in the same grade. For example, kids born from March 2018~April 2019 will be in the same grade at school.

In Japan, it is determined that all children born between March 2019 through April of 2020 will be in the same grade in school. If a child is born between these dates they will be in the same grade together. Therefore, a child may be attending school with another child that is slightly older or younger than their counterparts. They will all be in the same grade.

In Australia there is what we refer to as a 'cut off' for an age group going into a year level at school. This can mean that there is almost a year difference between the youngest and oldest in the one class. This is obviously the case in Japan as well. The cut off in Australia is June and I believe it is the end of March in Japan (I could be wrong). Therefore those born between March 2019 and April 2020 will all be in the same year level.