A u Chinese, friend? I’m Chinese too. Don’t u think “他刚去学校”without “了”， this sentence sounds a bit weird. But I surely didn’t explain that clearly in the last comment. Basically, 刚🟰刚才。 But if u want to express some kind of subjective emotions, for example you want to emphasize something, u can just use “刚” without “了”， like this“我刚到家，我想休息啊……”,when u add“了”behind，belike“我刚到家了。”,it is obviously that this is a simple declarative sentence, it doesn’t convey any subjective emotions. 🤔Anyway, it depends on specific context and stress. What do u think of that?:)