Like Sick says you will run into trouble if the tax man finds you are only working for one company. In other 'trades' this is catered for ie building industry (SE 60 I think is the form employers use).
That said I can name you lots of flying instructors who have worked for one company for years and no one has caught up with them. Im freelance and have been for the last 5 years, however I do work for several companies and have also instructed on a self employed basis.
If you can get the school to agree to it you could invoice the odd student directly for your time once in a while just so you have more than one customer in your accounts.
At the end of the day talk to an accountant about it.
I wish you luck with it, I can think of no other profession where an individual puts so much into getting qualified for so little return. Bring back the old PPL instructor route I say before the training world colapses!
[This message has been edited by Fly Better! (edited 19 April 2001).]