If students from the wrong year groups are showing then they'll need to be correctly assigned on the Classes page. Once it's sorted there, it'll be fixed on the OUMTC enrollment page.
To fix this:
Go to Account (bottom left) > Classes (or click here)
Tick the relevant students or classes and press Update (at the top) >Set Yeargroup> Year
Set the correct year group for all students.
Now go back to the OUMTC Setup page and the correct students should be showing.