I have been charging my standard fee for a constructed class of less than 15, + mileage. The class is actually 20+ but I do it for less to help the school and because I really enjoy it. If you do this on a trial basis, I suggest giving the fee you intend to charge and not a discounted “trial” fee. Not that its bad marketing but if its on a trial basis, they are going to be looking for reasons to justify that price. When the price is known, it appears to be a standard professional fee for a guaranteed professional service. Basically, if you give a discount, then keep it. If you don’t and they won’t cover it, then its because your time must be more valuable and they couldn’t afford you (at this point you can negotiate something suitable). When it comes to working with schools I’ve found it to be more effective to give a set price up front so they can work it into their budget in the future. If they don’t like your sevice, they simply won’t ask you back.