To find out the entry date of your Students per Year group, go to Reporting > Custom Report > New Report.
On the General Filters side, you would choose the Year you wish to see the data for by ticking the empty box beside that year i.e. 0.
You would then go to the Display Columns on the right hand side and using the tabs underneath click on the "Dates" and then tick the empty box beside Entry date, then click on the tab below - Run Report.
You can now see the Entry dates of the students in question. i.e. Start dates of Reception or Nursery in September.