You need to roll those students forward to the new school year. I assume you've created the 15-16 calendar? I would first print out an enrollment summary for that grade level/school, then you have an idea how many students are rolling forward.
Then go to System Admin, Student, Enrollment Roll Forward.
Select the 14-15 school source the data is coming from (even if the same school),
select the grade the source is (current grade level),
source service type (I select all three),
select source structure, Main
Select Start Status (if they are in the same school, choose 02, if coming from another public school in your district, select 11
select the destination calendar 15-16 (School Name)
Select Destination Grade Level (09, 10 etc)
Select destination structure, Main
Start Date Override - if you don't put a date in here it will be the first day of your calendar. I choose to make my calendar go from 7/1/2015 through 6/30/2016. Our start of school is 9/1/2015. So I put in the start of school here and override the 7/1/2015.
Hope this helps.