What Macs Should You Buy For A K-5 Mac Lab?
The biggest question in setting up a Mac lab is which Macs are going to be bought? A lot of this depends on budget.
If money is no object one might think I would suggest going for the biggest iMac screen and maxing it out. This would be wrong. First of all, the 27' iMacs are almost too big, in my opinion. The smaller ones will be fine. Also, keep in mind, the bigger the monitor the more space you need.
Ideally, I would go with the fastest 21.5" iMac with maxed out memory and the largest hard drive possible. You might not need the hard drive space now, but this will help make the computer last longer into the future with more space. The memory is not necessarily needed to be maxed out, but if money is no object, go for it. The more memory the better.
In most cases, money will be an issue and your best bet might be a Mac Mini with a cheap monitor. The Mac Mini is plenty fast for a classroom setting. If you look into a Mac Mini keep in mind that you have to buy keyboards, mice, and monitors.
Whatever you choose, make sure you buy as much memory as budget allows. Memory is the key for faster computers.
If you are just looking to get machines for internet, word processing, spreadsheets, and presentations you might even want to consider a room of iPads with one Mac as the base. The Mac (Mini or iMac) could be used for syncing files and printing (since printing is not officially supported on the iPad yet).