A legal exec is basically anyone in the law firm that has the legal knowledge to do lawyer-esque work but is not currently training and is not yet called.

Since it is April now, it is likely that you are in the middle of your TC. Finish your TC first. I don't think they can let you go on the basis that you failed Part B, but idk. It would also be really cruel to do that. You are allowed to complete your TC even though you failed Part B. Then, go talk to the partners and ask them whether they will retain you if you take Part B again come July.