Oh brother! Those bloody junior high boys. I had a funny interchange with a little group of 1年生 boys in between classes. It went something like this (translated because I love you):
Boys: Uuwah, Danieru sensei. Haroo.
Me: Hello there. What are you up to?
Leader boy: Nothing. Oh! Come, come.
Me: Where? (suspiciously)
Leader boy: To the joseiben! (Girls' toilet)+(Probably thinking, heh heh, doubt he knows that word)
Me: No chance! Bad! Danger!
Leader boy: Haha (damn he knew it), come on! It'll be grand.
Me: No chance, mischief maker (I am fond of using the word 悪戯者・いたずらもの which can mean prankster, mischief maker--> be careful, it can also mean loose woman--> mental note to use this only to males, haha)
Me: So what's your next class?
Leader boy: Sashikomi, sashikomi!
Me: Huh? What's this sashikomi mean?
Leader boy: Mwuhaha, SASHIKOMI, SASHIKOMI!! Haha.
Me: (takes out electronic dictionary) Sashikomi, eh? Surely this is a dirty joke (下ネタ).
Boys: HahahahHAhHAhaha. He knows 下ネタ!
Me: Insertion. さすが (just as expected)
Boys: **ROFLing at my disappointed (but slightly amused) face**
So 差込 or 差し込む meaning to insert, "thrust in" etc. So to all my fellow ALTs on the frontlines, if you hear sashikomi, get ready to ask, 下ネタですか (shimoneta desu ka?). Little buggers will get a kick out of it, and so should you, seeing their reactions.
Haha, until next time!