Ethernet Connection dissapears

My Daughter is using a Macbook at school, and lately her ethernet connection disappears altogether. She is able to connect to the internet via any Wi-Fi she is near but not thru the hardwired connection. Any ideas?

The physical connection or internal to the OS?

I would first try a different CAT5 cable. When the connection disappears, look at the switch or router that the MacBook is plugged into (unless it's a wall jack), see if the link light it on. Then go into System Profiler and see if the Ethernet port shows up under the Network list. Also drop into Terminal and try to ping a well known location such as Google.com. Also ping an IP address to verify that it's not a DNS issue, but if WiFi works at the same location I doubt it could be DNS related, but I've seen stranger things happen.