A group of high school students are transported to a castle when a promise is broken with a agent from hell. The Students must find a way out while defending themselves against many strange and deadly creatures.

I believe as written, your language is too generic to properly captivate the reader.

What will help is specifics

1) A group of high school students–This is as generic as it gets. First you need a lead character, however even if you decide it should be a group, be more specific, A group of high school football players, a group of high school mathletes, for instance.

2) are transported to a castle– A castle? A medieval castle, an English castle, a dark and scary castle. Is the castle on earth or in a different dimension?

3) when a promise is broken –What promise?

4) with a agent from hell.– An imp? a powerful demon? The devil in disguise?

5) The Students must find a way out– If the agent-from-hell can transport them to a castle, how is escaping going to help? He can just re-transport them. This seems like a flaw.

6) while defending themselves against many strange and deadly creatures–This isn’t too bad, in fact I would just cut the strange part, it isn’t needed “While defending themselves against the castles deadly creatures”

Remember this is what first drafts are for, to give a foundation that will be re-written and improved.

If you can be a little more descriptive and specific I believe your next draft of this logline will properly reflect your story in an interesting way.

Hope that helped, Good luck with this!