Need help with iMac network performance in a lab environment

I need some help troubleshooting performance issues I'm being told about for a lab of iMacs on a school network that I maintain (I work for the district IT department). The iMacs are hardwired to an Ethernet network. I love Macs, but I haven't really dealt with them on a regular basis for over 10 years.

The teacher reports that some days are just fine and other days there are frequent disconnects. On the network switch side (we're all Cisco), no errors show up, but network layer 1 and layer 2 drop for random machines at random times, for about 3 to 5 seconds. I can't see any cause for it on the network side, and I don't know where to look for issues on the Mac side.

These network connections were recently moved from a FastEthernet switch to a GigabitEthernet switch, but the new switch seems to be working fine.

We have other Mac labs on GigabitEthernet switches, but so far this is the only lab having issues.

 

The lab consists of 20 27inch iMacs from May of 2011. They are running Yosemite and they are connected with Apple Remote Desktop, but are not connected to a server with SMB.