It helps to have people onside. I have three DC, two with additional needs. One current yr 11 with full EHCP and diagnosis as long as your arms, the other yr 4 some diagnosis doesn't qualify for EHCP as working with school her needs are met. Her diagnosis is social communication disorder which mainly presents as severe anxiety (she physically shuts down/ freezes and is completely unable to engage when she gets overwhelmed. Sometimes she just faints) Most of the time she's a happy engaged child.
With my DD, yr 4, we have had a differentions with her peers from back in yr R. Working with the school she went into the class a few moments before her peers via reception so wasn't pushed and bashed around, she had a peg at the end of the row of coats, a draw for her work/ stuff in a slightly different location. She had the option to stay in at break time if she needed time out, she tried but wasn't forced to join in school swimming lessons (she can swim) because they were very noisy and everyone got very excited. She became very overwhelmed and it took two adults to help her out the pool. Because school have witnessed the extreme side of her anxiety getting the basic extra support hasn't been a massive fight.
DD has documented evidence of her strugles and a diagnosis which helps us get the support moving into each new year group phase.
An EHCP application is unlikely to get taken seriously if you aren't able to prove by documentation a track record of problems and needs not being met. I'm not suggesting you don't apply but suggesting initially you focus efforts on building a paper picture of needs and working out what support is lacking that you're requesting from the council beyond what school can provide. The EHCP supports a young adult through to 25 and post 16 support are starting to be more onboard with adhering to support needs so its not something to write off just because it wont be in place for September of yr 11.
DS1 YR11, autistic full EHCP major presenting difficulty is anxiety and need for rigid rules and structures to be followed. DS1 has also had a seperate entrance into school. His first point of contact has been within the autism provision he's attended rather than a bustly over excited class of teens. He then partook in a full academic timetable of subjects but we droped PE, he does PE from home i.e. we make sure he does some exercise every week, this enabled some downtime within the academic week to work through any issues in other subjects. For each subject he packs up and leaves just before the rest of the class so peek busy periods in coridors are avoided. His allocated seat in classrooms is to the side at the front so near the teacher for support and near the door should he need to remove himself from the class if overwhelmed.
DS has a place to withdraw to, a quiet supervised classroom in school. Not doing his subjects has never been an allowed option, by me or school, its a route I see others taking and don't know how you go back from. Once you start to get significantly behind in a subject its a mountain to climb to get back into it. I see it as a one way street. If that is the best option then take it but think of the end game, you can't partially drop a subject. You're better off focussing fully on 5, 6 or 7 than doing 9 part time.
If my DS is having an overwhelmed day he does his work in the quiet classroom. Sometimes he has to do it in an evening/ at the weekend if he hasn't been able to get the work before the lesson.
We dropped PE to create lag time in his week, he also didn't do all form group sessions - again creating catch up time. I did consider dropping RE but he wanted to keep going with it (he's very driven).
Dropping subjects without an EHCP is very difficult due to schools strict number crunching but saying you'll do certain subjects from home out of core school hours may be an option they would consider.
The three people to start by getting onside are your childs form tutor, the school SENCO (Special Educational Needs Coordinator) and the head of year.
If you haven't addressed your concerns with the school already, I'd start by sending an email to the form tutor and CC SENCO and Head of Year.
Detail the problems you were seeing with anxiety, the impact and I always find its useful to propose a few positive ways forwards whilst inviting their suggestions and recomendations. Include your contact details - email, mobile, landline and good times to contact. I say email because your message may be printed and you want them to be able to get intouch easily.
You say she is mentally unwell for fulltime school and I will try not to be too blunt but do you mean that due to her anxiety you don't know how she will cope (yes it is very overwhelming as a parent their anxiety is completely contagious and to resist the desire to keep them where they are safe and secure at home is a real daily battle), or you absolutely catagorically know she can't?
If there is a chance she can, the meassures to put inplace are that she is in school 5 days a week, every week but with pressure relief options. Different start/finish time to lessons, quiet classroom/ use of library space, academic curiculum only, emotional support etc.
If she is physically debilitated by her anxiety and sleeping/ shut down 20 hours a day then a part time timetable with medical support is something the school may consider.
What options are there and how do we go about exploring them? I'm feeling too overwhelmed right now to think clearly.