I have 20 years experience writing tools in PHP/MySQL and other languages. From searchengines to complex analytics tools.
For 2 years I taught English in China and wrote a similar system for students in my classes along side classroom games. How much information you want to store (for example on students, classess, teachers, lessons) and present (for example to head, secretarial staff, teachers, parents, students) etc etc and the manner in which you want to present/edit (for example online, on a local system) will have a large baring on cost.
Further details of your problem will enable me to provide a more accurate quote. I suggest if the system is not already well defined that this project is done in several smaller stages
I would be happy to discuss this project with you further.