Being in full time education dosn't exempt you from tax, you still pay it if you earn enough - ie more than the personal allowance of �5035.
If you only work in the holidays and don't expect to earn more than the personal allowance in the tax year you can avoid paying tax by getting a Form P38(S) from your employer. Fill this in and you won't have any tax taken out of your wages.
If you have a part-time job during term time, your tax code (probably 503L) should ensure you don't pay too much tax.
If you've paid tax and shouldn't (eg didn't fill the form in) your tax office should be able to help. At worst you'd have to wait until the end of the tax year to get it back.
If you pay any National Insurance, you can't get that back.