[bumbledraven]

> I then asked my teacher what I should write if I meant to convey that something was probably true, but that I actually wasn't certain or all that confident about it.

Not for school papers, but how about: "I guess [something]."

[ALittleLight]

I think the right choice is to quantify or bound what you mean. "The chance of X is 52%" sounds pretty confident and assertive. If you can't be that precise, then "For reasons X, Y, and Z this is likely, though not certain."

I think "probably" is completely fine though.