When you say "gone", i assume you mean physically not there. Has the residence permit that you had before actually expired? If it hasn't expired then did the school go and cancel it when you left?

Assuming that you don't have a valid residence permit already then, no, you won't need a release letter as you've already left the country. You'll just need to start the visa process from scratch with your new school. Release letters are only necessary if you want to change employers without leaving China. Of course you may need to get a reference from your old school but that's an entirely different problem.