it can be..um.... but um....it could be haha no2 is perfect. but for no1.. it's not that all. we can use it in special case. like your friend said to you as a kidding you are bad student so you are not going to school today and you can say it 나 오늘은 학교에 갈 걸? but it's a little awkward. usually we can use it listener dosent think but you think it will be happen or it is happening and then you can you it. like someone said to you is he in his room? but you know that he is in school. 그 남자 (not in the room) 학교에 있을 걸? or is he going to the park? and you can say 그 남자 (공원에 안가고) 학교에 갈걸? 그 남자 학교에 가고 있을 걸? like that. haha difficult to explain :)

-ㄹ 걸, which is -ㄹ 것을 is used to express a light regret about not having done something. - 학교에 갈 걸 그랬어요: I should have gone to school (but I did not). (그랬어요 is like a set phrase, and may be omitted without changing the meaning) - 버스를 탈 걸 지철을 탔네요: We should have taken a bus instead of taking the subway. - 집을 팔지 말 걸(요): We shouldn't have sold the house. They never have the sense of "I will probably ...". For that there is "..ㄹ 것 같아요", or "..ㄹ 거예요, etc.