Cheapest Mac is about $1,000, so not even close to within budget.
I have 2 of these:
http://www.newegg.co...N82E16834215057
You can't get that EXACT one anymore, but you can get the current model. Mine travels around the country with me on business. Doesn't weight anything, fits nicely on a seatback tray, doesn't get hot, sips battery juice and has tolerated being dropped.
You don't say exactly what you need it to do at school- at the < $500 pricepoint you're not going to get really good graphics or a powerful CPU, but unless you're doing graphics design/art/compiling/video encoding you won't really notice except when it's installing or updating. $500 just doesn't go too far in the world of laptops.
You can improve performance on these low-end laptops by investing $20-$30 into buying more memory. On those Acers it takes about 2 minutes to install RAM and is less complicated then changing some AA batteries (YouTube it. I swear.) Most of them ship with 2GB, but will support 8GB, and you can buy the 8GB module for $30. It WILL improve things.
You can also get more out of them by replacing the HDD with an SSD or by removing the Windows operating system and replacing it with a streamlined, light-weight Linux distribution. David's Acer is still running W7 and while the memory upgrade helped, it still huffs and puffs. Mine got a custom, ultra-lightweight Sabayon (Linux) spin and if you didn't look at the model #s you'd swear we had two different laptops. Also remember that the less burden on the system the less battery power it needs to sip. Most people run screaming in terror at the proposition of Linux but I figured I'd mention it since it can radically change your entire experience with cheap hardware.