If you work during the holiday periods (not term time), then you may be able to receive your wages without attracting tax. To prevent tax in the first instance is better and easier than to incorrectly pay tax and subsequently request a repayment.
You must satisfy the Inland Revenue that you qualify for this exemption and in order to do so, you must:-
be a student who is planning to continue being a student until after the end of the tax year.
not have total income from all sources in excess of your tax allowance. This does not include student loans, scholarships and educational grants.
Complete form P38s and hand this to your employer.