LAN world help

ok so here's what happened. My friends and I like to hang out together and open lan worlds at our school. We have school issued laptops and we all play minecraft on the network and it has worked fine up to now. we don't use mods or anything. Recently though we started to work on a RPG towny lan world and we all started playing. But as soon as we started finishing it up it started lagging alot and now when over 2 people join my minecraft freezes. Anyone have any suggestions on how to fix this?

P.S. I already tried changing video settings it has no effect.