First of all you need a van with a fitted system with plenty of water. 2 of us use about 1000 liters of pure a day to clean the outside of our school. It takes us about 3 days. We do specified internal as well which takes another 2 days.
Nobody can help you with quoting the school because the info you have given is very vague.
In no particular order.
You need to divide the school up into sections and try to estimate a cleaning time for each section. I did it by side and added them all together.
Schools are usually cleaned once a year so they are hard work. Some bird strikes are difficult to remove so if you think a window is going to take a minute to clean it will probably take you 2.
If the school is new then include a clause in your quote that you are not responsible for removing builders and paint residue. In fact that's probably a good clause even if it's not a newly built school as their maintenance staff don't always care about getting paint drips on the windows.
I would also try to incorporate a price revision every couple of years at least.
A professionally run school will ask you for a method statement and risk assessment.
Ensure you have an order number before you start work.
Stipulate that any additional work will be quoted for on the day and added to the invoice. You may need an additional order number for additional work.
If you get the job get to know who, how and when you will get paid. What do you have to do to get that invoice in the hands of the right people so you will get paid. Don't be afraid to follow it up.
Our local council pay the school window cleaning bill so we are dealing with another totally different identity. Our council is financial 'bankrupt' so they aren't in a hurry to pay.
They will possibly ask you to clean during the school holidays as there shouldn't be children around. After school activities in the holidays can be a problem for you.
We aren't allowed in the grounds on exam results day and over a long weekend when the town has a music festival and the grounds and car park is turned into a caravan and camping site.
Understand that the facilities manager at the school can't plan to save his life. Try to find out when they are pressure washing the concrete paving around the school. They usually do it after you have cleaned the ground floor windows.
If they are going to do this we have been known to leave the ground floor windows to the last minute.
You will have to work around other trades who will also be working. For example, an electrician repairing outside lighting and the alarm guys doing maintenance on door sensors.
I have a key to the grounds and all the internal gates so we can clean over weekends if we have to.
Here's how things can go wrong when quoting.
Our school has an atrium and an entrance atrium. We find that water gets inside the ali box sections and takes ages to drain out.
What I estimated would take us 1.5 hours actually takes us near 5 hours. The sections are bolted together but not sealed. Some box sections don't even butt up with each other. There are gaps where wash and rinse water gets through.
We have the advantage that the school atrium has a row of 20 very large glass panels. It helps us doing one complete row and then starting at the row underneath.
Spiders and spider webs are a major issue as well.
The same situation applies with the entrance atrium. That was overestimated as 1/2 an hour. It takes us 2 hours
because of the water runs. Often we have to go back and titivate the glass with a damp cloth and an Unger Fixi clamp on the end of one of our old Teleplus poles. We also do the entrance row by row and incorporate it with the other 'ordinary' windows we also have to do one row at a time.
Make sure you have liability insurance that covers you for the glass you are working on. One of those glass panes in the atrium exploded a couple of years ago. Thankfully it happened when we hadn't started cleaning yet, but if it had of then we could possibly have been blamed for it.
You will notice that the classroom windows are in 3 sections. As the top window bleeds down onto the window below we have to do them in sections. We wash and rinse the top window pane and do the rest on that side. We then start back at the first window and do the middle pane etc. If we clean them as one we leave streaks on on lower panes where the rinse water has run down.