Oh, your post made it seem like being near other people is what is making you particularly uncomfortable.

So if you can only focus while sitting still... then sit still and focus completely on your lecturer. If you need to be still but can't seem to focus... bring yourself something to grip tightly in your non-writing hand. Focus on it's texture and the warmth or coldness of it. Same with sounds - focusing on sounds can work if you're staying really still.

Or even focus yourself on the way your muscles feel. Are you tense? Are you relaxed? Do you have any body parts that have just fallen asleep?

You might miss a minute of the teacher talking while you focus on those things, but it should help knock you out of your daydream and back into your lesson so that you can refocus on what you're learning.