Break it down into areas, take your time and make sure you know what the customer wants done in/out etc..
Price by time ie how long it will take to do, bear in mind many schools and colleges only get cleaned yearly and the frames can often be in disrepair.
You should make a reasonable rate, don't expect a killing because there are always others willing to throw in a stupidly low quote, try to sell your quality service and local connections, but ultimately they are after the cheapest price
Darran