The guy who sits behind marsha likes to play the drums on his desk during class

The infinitive phrase is "to play".

An Infinitive phrase is a compilation of words comprising of an infinitive and the modifier (s) as well as (pro) noun (s) or noun phrase (s) which capacity as the actor (s), indirect object (s), backhanded object (s), or complement (s) of the activity or state communicated in the infinitive, for example, We expected to leave early. To Leave is the infinitive phrase that is present in the example sentence.