Have everyone return when you start the new year with full needs and cured medical/pastoral issues. This saves you from having to run the game during summer to top everyone off and cure backlogged issues while making it clear why staff can't train during the summer. 

If you wanted to make something more challenging to counter-balance this admittedly beneficial change, make it so you cannot buy new campus plots or build new campus buildings during the year... only allowing you to do that during summer. (you can still place objects and build/move rooms during the school year, just not the actual buildings. 

It would make the Summer look and feel like summer. The previously busy campus is dead and empty until the start of the new year.