How would you tell your Dad you got expelled from seminary school?

Dont. Play it off like you graduated early and did so well that they want you to do some undercover work.

"Dad, I got expelled from seminary."

("Seminary" means "school for clergymen" -- "seminary school" would mean "school for clergymen school," which phrase makes no sense.)

See John 8:32. Change the last letter in the verse to a "T" and it will describe your situation!

If there's a genuinely good reason, maybe it won't be so bad just to tell him outright.

However, I'd definitely pray about it and maybe try to get some plans together of what I would plan to do now.

If you learned anything while you were there -- then you know that the truth is always the best approach.

That being said, you can always say that decided to leave because you don't want to be a pastor (which is -- I assume -- the truth).

well, you should tell him straight, maybe give him some cushioning first, by making small talk before hand

Father ... I'm no father, nor will I be ...