The very worst (seriously- worse than making suggestions about the night-time activities of their mothers) insult used by pupils at the primary school in Salford where I used to teach was

Your mam shops at Netto!

(Netto being the local downmarket cheap supermarket- whereas it was acceptable to shop at Kwiksave, also downmarket and cheap...)

If their mum put their PE kit in a Netto bag to bring it to school they'd lose the bag in the bushes rather than be seen carrying it :0)

And when we took a party of kids to the seaside at Prestatyn, my colleague inspired them by saying
"Last one in the sea shops at Netto!"