I had the same doctrine when I was young (though at Sunday School, not at High School).
Eventually it seemed to me that "theirs is the only truth" was the most incredible part of the doctrine.
but when it comes down to being metaphorically hit in the face with them every day
Well, I hope that school teachers are not violent -- that "hit in the face" is only a metaphor now.
What would one offer as advice to not sway from the Dhamma by distorted teaching
There's some Buddhist advice about how to study Buddhist doctrine in DN 16, called Four Great References, which describes how to listen to someone who is trying to teach or to re-tell the doctrine:
Then the Blessed One said: "In this fashion, bhikkhus, a bhikkhu might speak: 'Face to face with the Blessed One, brethren, I have heard and learned thus: This is the Dhamma and the Discipline, the Master's Dispensation'...
"In such a case, bhikkhus, the declaration of such a bhikkhu is neither to be received with approval nor with scorn. Without approval and without scorn, but carefully studying the sentences word by word, one should trace them in the Discourses and verify them by the Discipline. If they are neither traceable in the Discourses nor verifiable by the Discipline, one must conclude thus: 'Certainly, this is not the Blessed One's utterance; this has been misunderstood by that bhikkhu — or by that community, or by those elders, or by that elder.' In that way, bhikkhus, you should reject it. But if the sentences concerned are traceable in the Discourses and verifiable by the Discipline, then one must conclude thus: 'Certainly, this is the Blessed One's utterance; this has been well understood by that bhikkhu — or by that community, or by those elders, or by that elder.' And in that way, bhikkhus, you may accept it on the first, second, third, or fourth reference. These, bhikkhus, are the four great references for you to preserve."
So -- "without approval and without scorn, but carefully studying ...". You did say you're doing something like that, "I try my best to have no aversion toward other followings & I also take a genuine interest in them".
Learning more than one language can help you to better understand each language, the same may be true, that understanding Catholic doctrine (including how it's taught and practised) adds to learning Buddhist doctrine too -- including what they might have in common (both good and not so good), and how they're different.
How do their lists of virtues and sins compare, for example?
And this isn't the place to discuss Catholicism, but since you mentioned "if one does not believe so they will perish", there's a slightly different statement now in Lumen Gentium published in 1964 -- see e.g. "Possibility of salvation outside the Church" -- though you wouldn't know that from the way they teach Catechism.
while at the same time not have hatred in general (due to their delusions & spreading of so) towards other paths
There's any amount of standard advice, I don't know what to say.
Just "turn the other cheek", and "love thy neighbour".
Or tell yourself (SN 35.88) that,
'These ... people are civilized, very civilized, in that they don't hit me with their hands
Or try to find a better teacher, or to communicate better with the ones you have.
Or more practice and less theory, if that makes sense.
And "hatred in general" sounds like a mistake to me, I think there are forms of Christianity, and individual Christians, who can be good friends -- even if they're possibly not the ones teaching that theology class -- which it would be off-topic to talk about here, but maybe just try not to generalise.