Your main character should be involved in the outcome, climax or conclusion of your story
If your character sits through the climax, observing the action, or not even participating at all, then they don't deserve to be the main character. Build up the storyline so they can at least learn a lesson, or come out of the conclusion as a changed person.
Don't force romance into your storyline
It's best to observe how romantic relationships are organically formed in reality. If on one page, your main character is single, and on the next page, they're getting their face sucked by someone who the reader barely knows, the immersion in the tale is likely to fail.
Don't base your story on previously established works of fiction
Popular examples include; the brothers Grimm, Hans Christian Anderson, Bram Stoker, Mary Shelley, and anything in the King Arthur universe. This is heavily overused by young writers and it has become quite a severe cliché, which your teachers will immediately recognise. However, if you are already an established writer, or a casual fan-fiction writer, people will be less likely to criticise you. There may be other exceptions to this rule, so feel free to comment them :)