Q: Remote Desktop will not allow screen share

We have two 32 station iMac labs (21") with a 27" iMac at instructors stations.

Both running Yosemite 10.10.2

Both running Remote Desktop 3.8

I can lock student computers, and I can control student computers.

I cannot share my screen with student computers.

When I attempt to do this, a Remote Desktop icon appears on the student dock, and nothing else happens.

Anyone have a suggestion? I've tried changing source computer and source display and nothing changes.

Thanks.