How good are free antivirus software?

I know that when it comes to antivirus software licensed is the way to go but I might get a pretty big tender soon. 30 laptops that needs antivirus software. This will come out quite expensive for the school especially if they need to renew every year. The only other option is the free option like avast, avg free, commodo, ms security essentials etc. How good are these free anti virus software and do they really do a good job at protecting a PC? Or will it be better for the school to buy licensed AVs? I am quite a fan of Kaspersky and AVG