I really hope they don't make you do something you don't want to, so I hope that is sorted out. Doing 5 A levels is very tough, a few people in my year are going in for it, but it's not recommended... However, if you do go in for it/ end up doing it, I really think that you would be fine. It would likely be a tough few weeks, but as you can speak it fluently, I think it's a case of learning what the sounds look like written down, if that makes any sense. Also, you must bear in mind that a foreign language AS is nothing like an English Language AS/GCSE, the heart of it is being able to communicate clearly about certain topics (TV, health, sports, holidays etc). Once you get those reading and writing skills, you've nailed it. There is somebody in my foreign language class who was fluent in speaking, but had never learnt how to write it. Their writing skills are good already, and as no effort is being expended on speaking and listening, time can really be focused on what is needed.
I hope your form tutor comes to their senses and stops trying to force you!