hi ana,
i know how you feel. i am in that situation now and have been for 2 years. big classes and mixed levels. i used to be very frustrated by it because i saw a much more effective and effecient way to teach them but the school won't match the student's by their levels. so knowing this was out of my hands i tried to relax and have some fun with them. my feeling is there is no sense banging my head against a wall that is really hard and won't budge. it starts to hurt after a while. the school has imposed some ridiculous situation on you so that can't expect miracles.
that being said, there are a few things you can do to help ease the pain. use groups as much as possible. and allow the groups to work together during the class. and change the groups up every week. don't keep the same groups as they will get stale.
if you are using games in class, which i highly recommend, use games that involve chance for points, not skill. this way, the low students can earn points just as easily as the high students. chance games could include card games, rock scissors paper, rolling dice. also, charades and pictionary don't rely too heavily on english ability.
i hope this helps.
also, here is a great
list of games to play with large classes.
eric