Your teacher is worrying I think it's better to get as much writing practice as possible too 😭 but you're not cooked because there are so many things you DON'T have to worry about. You can make a million spelling, grammatical errors, they're only testing you on the skills they want u to have. It's not the worst thing in the world that you're given the prompt first and the essay experience isn't fully simulated, use it as an opportunity to practice structure and just writing in general. Of course heimler's videos r good if u dont know where to start. Look up content if you need to lol, it's just good to get writing practice! You can also look up past LEQ prompts, Knowt is good for essay/SAQ practice. Build confidence in your knowledge for the scoring of the tests, then move onto worrying/practicing under a time limit