As previously said, it all depend on the concurrent active connections, the traffic type and bandwidth in use.
You can start by deploying only 1-C9105 per classroom, monitor it, and then decide whether if it would be necesary to add more APs or not. Another interesting deployment would be to stagger APs to both sides fof the main corridor and add a couple of additional APs at the end of the rows, or another similar scenario where you could provide redundancy and split connections into classrooms between multiple nearby APs. By the way, don't forget to use @20MHz channel widths so to avoid channel overlap and increase capacity.
