My name's Greg and I'm going to give you some help with your question.
As a general rule parents have financial responsibility for their children until they reach 16, following which they can lawfully choose to leave school and enter employment. From this point, your obligation to pay child maintenance will depend on what your child chooses to do. Your maintenance payment will continue if If they choose to continue with non-advanced full-time education for a minimum of 12 hours a week. This includes, for instance, GCSE's, A levels, NVQ level 1 - 3 and BTEC National Diplomas/Certificates (although not the Higher variety) but does not include university degrees. Payments will not then come to an end until they hit 20 years old (i.e. the final age during which you wil be liable is 19) or they finish full time education, whichever is sooner. The fact that she has a part-time job is not relevant if she is in full time education.
If you're unsure about whether her studies meet the criteria for non-advanced education, it might be a good idea check with the CSA because some course (e.g. apprenticeships) do not count.
Let me know if you need anything further.