It would depend on your school. At my secondary school it was compulsory to do a B language and if you weren’t good enough to change to a new language at Ab Initio level. However, when I moved to the UK, my sixth form tried to force me to do ab initio because I wasn’t fluent in French so it really depends on how strict your school is about following the IB rules. I refused to switch and continued with French B because I had done French through middle school and felt it was the ethical thing to do. It did take a lot of work, but in my case it was worth it as I improved so much and have enjoyed putting it into practise during my trips to France post graduation and will possibly even complete a French module as part of my medicine degree. I would speak to your French teacher and IB coordinator to see what your options are