There is nothing wrong with your sentence.  It contains a participle phrase which can be fronted:

Hiding her things and dumping her backpack in a muddy puddle just outside school, the boys had been bullying her.

As to the dash, I would not use it and would opt for a colon (:).  The colon signals that the following clause further explains the preceding clause. (The Art of Styling Sentences, Ann Longknife.)

"The boys had been bullying her again : They hid her things and dumped her backpack into a muddy puddle just outside school grounds."

I prefer the version with the colon.