proof of full-time education?

What is considered acceptable proof of full-time education if, for example, I wanted to prove that I am eligible for child maintenance despite being over 16?

I've tried to use my college ID card previously, but they seemed unsatisfied with me using it as proof of full time education, claiming that I could use an old ID from the year before or apply for a new one then drop out of college. I thought this was fairly unreasonable, but regardless I will give jelly1000's second idea a go.
Thanks!

Dont college ID's usually have the year on though- e.g. 2016-2017

What is considered acceptable proof of full-time education if, for example, I wanted to prove that I am eligible for child maintenance despite being over 16?

yeah what jelly1000 and Tiger Rag said should defo work. I had my school do the same for me as proof of full time education, and it worked a charm for the child maintenance thing. The student support at your school should have quite a bit of experience with regards to what needs to be written on the headed letter to give it some bona fides.

On the off chance they don't however, as I recall (albeit this was a letter I glanced at for all of a few minutes, many a month ago) mine went something like "I (teacher person) from (your place of education) am writing to confirm that (you) is currently in full time education which he/she started on (start date) and will finish on (end date) studying (xyz). If you have any further questions or require any further proof of (your's) full time education status, please do no hesitate to call on (teacher person's school number). sincerely (teacher person)"