Armando no había a la escuela antes de que su madre me llamó (volver).

The right conjugation of the verb "volver" is:

- Armando no había vuelto a la escuela antes de que su madre me llamó.

Explanation:

The translation of the sentence is:

- Armando had not returned to school before his mother called me.

At the time of conjugating a verb, regularly the first thing that must be identified is the tense in which the sentence or text is found, in this case, since the verb "haber" is presented, in its form "había," the tense is perfect compound, which is characterized by having its second verb with endings "ado" or "ido," but in the case of the verb "volver," the right conjugation form is not "volvido," that form doesn't exist, in change, it is an irregular verb whose form in perfect tense is "vuelto."