“The Irregular at Magic High School The Movie: The Girl Who Summons the Stars” is the prequel to this. The prequel to the movie is the first season.
Mahouka Koukou no Rettousei: Raihousha-hen
Shiba Miyuki's classmate Kitayama Shizuku is on her way to study abroad. For magicians, this is normally impossible, since allowing the genes of someone who can use magic outside their home country's borders is tantamount to giving up national secrets. But it's allowed to happen in one case-exchange programs. And that's how Angelina Kudou Shields has arrived in Japan from the USNA to study at First High. Around the time Lina arrived in Japan magicians began getting attacked by a mysterious being who leaves his victims drained of blood. Will Tatsuya be able to discover the identity of the "Vampire"?
Actually, the official site does not say anything regarding that, trackers claim that the movie is a sequel to the second season so far. It will remain as it is and it will be monitored for changes.
I have watched both the first season and the movie and the season which is airing right now picks up where it left off in the movie.
I checked and some sites including AniList do state that the movie isn’t the prequel to the current airing anime, I think so this is because of the events occurring differently in the light novel. So, don’t change anything and also add the movie to relations under the second season’s anime page.
I think that movie is rather a sequel cause in movie they did know Lina and they knew that she’s STARS agent. In this (second) season at start they didn’t know that. But that’s only my speculations and I can be wrong.