Correct, the sentence is already correct. Had attended
Past perfect tense. Had + past perfect. Present perfect tenses have the structure of: has/have + past participle of the verb. Past participles usually end in -ed, except for the irregular verbs.
In comparison, present tense of verbs usually comes with the morpheme -s or -es. Sometimes, they retain the base form. This depends on the type of the verb. Past tense of verbs usually end in -d or -ed, except for irregular verbs. Lastly, past perfect tenses usually has the structure: had + past participle of the verb.