There are so many things to consider in determining a percentage to give back. Are you giving back a percentage off the top? Or on your NET? Careful what you promise a client!

I would be hesitatant to even start at 15%...AND I would be offering that on
NET and
NOT GROSS sales.
Not making alot of money and no assurance of future revenues from this place is reason to DECREASE the amount you are paying back. I reserve the higher percentages for clients that are repeat and bring me alot of work. The more they bring the more I am willing to cultivate that relationship.
In fact, I would try and offer a service (design of the annual etc.) that I mentioned earlier as a way of paying them back over an actual percentage and cash. (If this school held ANY opportunity of real revenue, one of the big companies would be shooting it and giving it a percentage back.) Remember that, and in your negotiations be sure they are aware that your decision to do this for them is actually to their benefit, as no one else has offered ANYTHING. (Assuming this of course, as they are looking for someone and talking to you.)
The reason more photographers fail, is
poor business decisions. NOT poor photography. If you want to make money at this, you MUST realize, you are in the BUISNESS of photography and your decisions must reflect that.
My mention of Lock-Out contracts was only to point out how the Big companies insure they make a profit. Lifetouch/Prestige is only one...MANY others do the same thing.
And finally, it might be a job you consider taking and only break-even on...
IF you feel the potential for building new clients is there. I do a couple of private school every year, and I don't make alot. BUT, I do clutivate alof of new clients that spend money on other work at my studio. (Or otherwise I wouldn't be doing the work!)
CJ