Okay, your English teacher told you not to. But is there a rule anywhere that says we can't start a sentence with but, or and - or so for that matter?
I haven't found one, just assurances from other professional writers that there isn't one.
Let's acknowledge that your English teacher's advice was useful when you were starting out. Young writers might be tempted to start every second sentence with but, and or so.
Some myths of writing began as good advice and morphed into inflexible rules. The advice about never using but is a striking example. (See: In praise of but)
We should reserve sentences starting with but, and and so for special occasions.
Use them when you want to emphasise a point. Otherwise use a comma.
'The advice was useful for young writers. But now many people think it's a rule of writing.'
'The advice was useful for young writers, but now many people think it's a rule of writing.'