Came to be studying with us with 〜ことになる

先生(せんせい):

「今日(きょう)からみんなと勉強(べんきょう)する事(こと)になった、灰原(はいばら)　哀(あい)さんです！」

“This is Ai Haibara. It came to be that she'll be studying with us starting today.”

「みんな仲良(なかよ)くしてあげてね！」

“Everyone be friendly with her!”

Key Points

1. 〜ことになった = it was decided that

   • Here the teacher is announcing an arranged new situation.

   • Ai has not “accidentally ended up” here; the school has determined that she will join the class.

2. みんなと勉強(べんきょう)する is the content of that decision

   • The full chunk means “to study together with everyone”.

   • ことになった wraps that whole action and turns it into the new arrangement.

3. Starting today is part of the announcement

   • 今日(きょう)から sets the starting point for the new situation.

   • The teacher is formally introducing Haibara as a classmate from now on.