もう学校始まってから二週間もたってるよ
Please verify because I've seen in some dictionaries it's already an obsolete word, it means mota-"having done"
Some dictionaries don't have a word mota so I can't see the conjugations on this one.
「たって」 is from the verb 経つ{たつ}, meaning "to pass (time)". It's usually not written in kanji.
も is a particle attached to 二週間, and when attached to a counter it means the speaker believes that that amount is a lot.
もう学校始まってから二週間も経ってるよ
It's already been two weeks since school started!
You are parsing it incorrectly: 二週間もたってる is 二週間も + たって（い）る.
も is sense 一・④ in 大辞林
も
一 (係助)
[...]
④ 極端な物事を指定し、強調する。…さえも。「聞いたこと–ない話」「太っ腹の社長–、今度はまいったようだ」and is for emphasis.
- たって is the te-form of 経つ (as in 時間がたつ "time passes")
