If you don't have domain knowledge or a partner with that knowledge I'd suggest staying out of it.
(the following assumes public school. Some points are specific to public schools, not private ones)
If you are asking on a forum like this, I'd guess you should sit this one out.
If you already have the software and infrastructure based on another vertical market, then you can probably leverage that expertise. If you are just starting out, it is going to be an uphill battle.
If you are up for the challenge, good luck. Let us know how it goes.
among other links from google
School Tool is an Open Source School Management System.
The vast majority of schools in the UK use SIMS.net (School Information Management System) from Capita.
BECTA (which used to be called the British Educational Communications and Technology Agency) holds online functional specifications for what (they say) School Management Systems should be able to do, here.