Don't be sad, it's a great mark
It's only year 11 and it's a step up from year 9 and 10.
Tips:
Use flash cards for new vocab and regularly revise them
Practice ur vocab on a website called 'Language Perfect', but u need to be member and ur school needs to sign u up.
Answer a wide variety of questions and get ur teacher or a native speaker to mark them and give u feedback
Read or watch movies in Japanese and note any new vocab or good expressions u come across and find the meaning
Memorize some idiomatic expressions
Good luck for ur future tests and I'm sure u'll get 100%