the subject is "she" which is the 3rd person singular. present tense verbs in english only change for 3rd person singular by adding the {-s} so it would be "sings"

Q: She is not the only girl who ____ well in class. sings or sing? A: sings I thinks this pointer would help :) Do not be misled by a phrase that comes between the subject and the verb. The verb agrees with the subject, not with a noun or pronoun in the phrase. Do not be misled by a phrase that comes between the subject and the verb. The verb agrees with the subject, not with a noun or pronoun in the phrase.

Yes, sings. There is just one girl.

Randy and Lee-ssi are correct. To use 'sing' the sentence would be: "They are not the only girls who sing well in class." Use 'sing' here because 'they' is plural.

sings!!!