If it's your sole source of employment, the employer is on slightly dodgy ground, but if the pay is ok for you and the operations fairly legit, then thats nothing for you to worry about; (potentially you could claim you thought the income was net and invite the IR to extract the tax from the employer retrospectively, and not from you; not that I necessarily recommend it!).
All you have to do is produce invoices for the school, and the self assessment form becomes a bit more of a headache.