What's the frequency of cleaning.
How long would you estimate it to take one man (oops, er, person) to do?
Any special requirements?
It's really impossible (I think) to help you without us seeing the building.
I'm guessing (because your name is 'Sue') that you have the cleaning contract for the school and academy; however wish to sub-contract out the window cleaning.
Is this correct? If so, why don't you phone three or four local window cleaning companies and ask them to quote.
Modified later:
Sorry, Sue, looks like you're a fellow window cleaner.
I think what you've got to do is estimate how much time it's going to take you to clean the place and price accordingly. It's difficult I know, especially if you're new to the job. Just work your way round each area and guess the best you can for how long each part will take. Then add it all up and calculate how many hours/days it will take.
Don't do what I did with a large building; quote cheap; because I ended up dropping the job six months later. Be sensible and if you wish to earn £200.00 per day (I don't mind), and you think it'll take you two days, quote £400.00. If you're not bothered whether you get the job or not, quote higher!
At least you've only got one point of call for payment; unlike residential work, where you've got loads!
I doubt anyone could help you more, without more information.