It wouldn't be the first time teachers are wrong - we're all engineers doing this for real. It is possible under some circumstances to drive a small relay with a single transistor, but if the load is more than a few milliamps, a standard 8052 can't supply the base current needed. You could use a darlington, or a darlington connected pair, or Mike's concept, or a ULN2803 or ULN2003

Steve