Email a department head as student recommending a professor for a tenure position
Of course you can write to the head. Make it a formal, printed, mail, however. Email might just get mislaid. But also note that most tenure decisions go through committees. It might be more appropriate to write a formal letter to the head of that committee. 

The letter on paper will become part of the tenure file to be discussed within the committee. 

Also, say more than that you support the professor. Say why. How has he/she helped you or others, for example. Inspired you. Whatever.