Meaning of ~たことにする and もんだ

Question

I found this sentence in a short story:

「先生の口から『どうしたらいい』なんてセリフが出るとは意外でしたよ。僕はてっきり、子供たちには好きなようにさせてやりたいから、ほかの先生どもがぎゃあぎゃあ言ったときには、以前みたいに僕が子供たちをそそのかしたことにしてくれって頼まれるもんだと思っていました」

As far as I understand, the speaker - the viceprincipal of a school - due to a teacher complaining went to a classroom to check about an issue that teacher was having with the students, and found them with cactuses on their desk, to be studied as graduation project because they have supernatural abilities (「だって、サボテンには本当に超能力があるんです」).

I found that 「short past + ことにする」 means to act as something false were true, as confirmed also by this question; as far as I can tell, this is the translation:

It's unusual to hear a teacher say something like "What should I do?", right? Without doubt, since the kids want to people letting them [させてやりたい] do what they like, when the other teachers were complaining, I thought they were asking me to act for their sake [てくれ] as if I insigated the kids like before

but I'm not really sure, since it sounds odd, and I have some doubts:

• 以前みたいに means "like before", like he already did something like that before? Alternatively it could mean "previously" as in the viceprincipal did that before the students brought the cactuses, but if that's the case I don't understand why the 「みたい」.

• what about もんだ? I know it's used, after the dictionary form of a verb, to state something that's expected or a social norm; does it mean the other teachers are asking that like it was something expected from the viceprincipal?

• is my understanding of 「そそのかしたことにする」 right? I can only think, if my translation is right, that the other teachers want the viceprincipal to act like the cactuses were an idea he gave to the studends, like to cover the fact that it was something the students want themselves to do, but I'm not really sure about this.

Answer 1

You have several mistakes parsing the second sentence.

As the start point, もんだ is the colloquial form of ものだ, which falls under the following usage in the dictionary.

㋒（「…するものだ」の形で）それが当然であるという気持ちを示す。「先輩の忠告は聞く物だ」「困ったときは助け合う物だ」

Thus, the word should be translated like "as a matter of course that —". Meanwhile, てっきり is an adverb "doubtlessly", but particularly used when one has a false belief.

１ 確かだと思っていた予想・推測が反対の結果となって現れた場合に用いる語。きっと。「今日はてっきり（と）晴れると思ったのに」

So, you see its function basically overlaps with もんだ and forms a seemingly redundant expression. Such kind of adverb is called 呼応の副詞 "adverb in concordance" in Japanese grammar, which has a certain practical significance, that it often signifies a start of encapsulated content, that will be closed by its counterpart. In this case:

僕はてっきり₍₁₎、［⸻］って頼まれる₍₂₎もんだと思っていた₍₃₎
I totally₍₁₎ assumed that₍₃₎ I was asked that₍₂₎: ⸻

The whole content inside is a reported speech, which is grammatically detached from the outer part.

Now,

子供たちには好きなようにさせてやりたい
[subject] wants to let children do what they like

You translated this part "the kids want to people letting them do what they like" (which would be 子供たちは（人に）好きなようにさせてほしい), but that is not possible, because 子供たち marked by に is the causee in this clause. Causation of intransitive verbs via command/instruction changes the marking of the original agent from が to に.

子供たちに好きなようにさせる ← 子供たちが好きなようにする

Your other questions struck me like you were distracted too much by the supernatural cactus. Although it's somewhat shocking, as I understand correctly, this passage as such has little connection with it. In general, Japanese teachers tend to be managerialistic that don't like children that don't listen to them or do weird things. I think what they are talking about is that this teacher vice-principal rather wants children to behave freely and might have sought support of the speaker before.

The full translation for your reference:

Edit: According to your new image, this is actually the teacher's line spoken to the vice-principal, which reads:

「先生の口から、『どうしたらいい』なんてセリフが出るとは意外でしたよ。僕はてっきり、子供たちには好きなようにさせてやりたいから、ほかの先生どもがぎゃあぎゃあ言ったときには、以前みたいに僕が子供たちをそそのかしたことにしてくれって頼まれるもんだと思っていました」
"'What should I do?' is such an unexpected word to hear from you. I totally assumed that you came to say that you liked to see children do as they like, then to ask me to act as if I put them up to do it just like before, when other teachers should make a fuss."

(The indirect speech part is continuous, but I couldn't find a nice way to reproduce it in English.)

Also, as a minor point, you translated 先生の口から as "a teacher say...", but this 先生 is almost certainly a pronoun that points to that specific person. So it should be "s/he" or at least "such a teacher".