Quite a few flying schools I've worked at prefer you to be self employed, but still expect you to teach soley at their school (even if they have no flying for you). They just want to get the benifits of an employed instructor with out having to pay the extras (ie employers NI). I have a friend (who has since hung up his wings) who was self employed (as every instructor is at this flight center), one particular day his normal school didn't need him so a rival school asked him to work for them for the day instead which he did. His normal school 'sacked'?? him for not being loyal to the flight center???
If you know the answer send it on a post card to blah blah blah.
Money, schools will pay what ever they want self employed or not. Of course it depends on how desperate they are, but watch they will still try it on.
The only plus side to being self employed is that you can claim back tax on material, courses etc which you need for your company. But do it right and get advise.
Good luck...