'teacher's desk'. Even if many teachers use that one desk you use the singular form 'teacher's desk'.

Ah ha! Greg is right if all you're doing is referring to what TYPE of desk it is: A student's desk, a teacher's desk, etc. However, if you're describing WHO uses the desk, it then depends on how many teachers are using it. If it's only one, then you use "teacher's" desk. If it's more than one, you use "teachers'" desk. And, don't forget if you are referring to more than one desk, you still use the same form: One teacher with many desks, "teacher's" desks. Many teachers with many desks, "teachers' desks." The rule which works most of the time is that the apostrophe comes after a plural noun. However, if you're referring to a person whose name ends in "s," (like James), it would be James' desk (NOT Jame's). Got it?