-IR verbs belong to what is called a "regular conjugation," by which is meant simply a large number of verbs that have the same endings (or are "conjugated" identically). Recall that you have already learned two regular conjugations, -ER verbs and -RE verbs. In the present tense of the indicative, these forms are, for instance:

Parler
Je parle     Nous parlons
Tu parles   Vous parlez
Elle parle   Ils parlent

Attendre
J'attends     Nous attendons
Tu attends   Vous attendez
Il attend      Elles attendent


-IR verbs too have regular, predictable forms. So, for example, the present tense indicative forms of the verb réussir would be:

Je réussis     Nous réussissons
Tu réussis     Vous réussissez
Elle réussitt    Ils réussissent

The present tense indicative forms for maigrir would be:

Je maigris    Nous maigrissions
Tu maigris    Vous maigrissez
Il maigrit      Elles maigrissent


To form the past participle of the passée composé for -IR verbs,simply drop the -R from the infinitive. So:

J'ai réussi    Nous avons réussi
etc.

J'ai maigri    Nous avons maigri
etc.

Notice that in the third question ("Au lycée, à quels examens est-ce que tu réussissais le plus souvent"), the verb is in the imperfect indicative, "réussissais," ("you did well [repeatedly]/you used to do well"). Remember that to form the imperfect of regular verbs, you simply drop the -ONS ending of the first person plural indicative ("réussiss-ONS") and add the imperfect endings (-AIS, -AIS, -AIT, -IONS, -IEZ, -AIENT).