In my opinion. 去 denote that you just do something, complement some sort of task with any tone, or emotions, But 要is different. 要 denotes that you need to or had too, there are too situations for this word. the first situation: you are forced,because you have to, you do not have options. second: you own needs. for example.我要吃饭，不然我能活。（I need to eat otherwise, i could not survive) I think you can undestand the differences between 去 and 要 in the following contexts, when someone makes noise that affect you and you can not concentrate on your work, 1 : 不要吵了，我要写作业 knock off ! i need to do my homework. when your courses are very important for you, you can not quit it for it's great importance 1 A：能陪我去书店吗？ B：不行呢，我要上课 A：Can you go with me to the bookstore? B: sorry. i can't I have to go for my course with respect to 去,there are the following scentences which would help you understand it 1 A: 你去哪里 B： 我去上学 A：where are you going? B: I am going to school So 去 here suggests the person do something without making an emphasis on your mood , your needs, as well as the importance of the thing the person has to do. so could you undestand the differences?

Both 去 and 要 have many different meanings in Chinese language. You can check this in a dictionary. "我要上课" is a ambiguous expression：it can mean i will go to class，and it can also mean i want to have class。which meaning to pick depends on the context。

ps '我要上课’can both show your feeling that you want to but not 'going to'later,or sometimes it might mean that You will go to school later.I think it is better to translate 'I will go to school' into '我要去上课了‘ in that case.

Yes,usually it is. e.g.‘我要去’+location=I am going to somewhere； ‘我要去’+做某事=I am going to do something.