I think it is a very common thing in year 11. I know 5 children (4 friends' dc and my own DD) who have issues in year 11.
It may be a mix of several things, pressure because of gcse, peer problems, identity crisis, first boyfriend/girlfriend issues and anxieties, fear of not doing well, general brain-restructuring issues which make them confused/reckless about almost anything, etc.
I wish you a lot of patience, I certainly had to watch what I was saying and also let go a bit.
Was hard when DD locked herself in because she didn't want to go to school, but it only happened once - so far.
They are making crucial decisions about their future (e.g. choosing A level subject which influence uni courses) and are a bit scared.
I calmed down a lot and have put the responsibility into my DD's hands. Of course I am here for her, and she can rely on me for help, but she has to go to school, I can't make her. She'll be 16 soon, I was very independent at that age, have to remember that.