actually, it omits some word in the sentence, 我要去學校. want 我（想）要去學校 should 我（應該）要去學校 will 我（將）要去學校 但相較之下，其實「要」才是可省略的詞，否則就會造樓主提到的語意不清。 我想去學校 我應該去學校 我將（要）去學校

You have to distinguish among them by context: My family cannot affort it but 我still(想)要去学校； I cannot play cards with you because 我要（have to）去学校; Tomorrow morning 我要去学校。 Chinese is a not-very-accurate language and sometimes even ambiguous. So do not care too much at this. Take it easy.

要在语境中理解，不要死记规则。

I want to go to school means "我想去学校。" I should go to school means "我应该去学校。” I will go to school means “我要去学校。“ I wish it's helpful.