One in 10 students in my class is / are rich.

Which bold word should I use?

Thanks.

Welcome, Lionel.

Is - the subject of the sentence is 'one student', therefore the verb must be singular.

Unless you feel that you're not really talking about one student, you're talking about 10% of the class.

According to MWDEU, the plural verb is more common in this construction, although the singular verb is found. Use of the plural verb is due to notional agreement - that is, the subject is seen as a percentage, as Faldage says.