Dress smart, go to reception and find out who is in charge of window cleaning. (usually the head or the bursar although this may be delegated until the final decision to the school secretary) send a letter explaining your credentials and inform them you'd like to make an appointment to offer your services.
One day after the postie should have delivered it (or you deliver it by hand) phone up to make an appointment. When you phone find out the frequency of cleaning at present and if an annual contract is awarded or whether it is done on a more informal basis..
If you get to see the man (or woman) explain your services and if all goes well offer to quote for the next clean (or from contract renewal date).
Find out what is required (you may be asked to recommend what is required) and give a "ballpark price" from your working sheet of paper. (ask how they feel about it) If they are happy with it then put it in writing adding a little. If they baulk at the price ask what they had in mind. If they cough up a price and its do-able and you want it then say "If I can get close to that can you give me the go ahead now? Because then we can schedule it in to our dairies"
Find out whether payment is from the school or the County. ("If I send an invoice
do I send it here? And how soon are accounts settled?) It could be paid by return or 30 days nett
Deal done! (In theory!)
Some times I'm brilliant, sometimes I'm merely very good, today I'm average.