Is it possible to share my internet from Wifi to wifi. Act as a wireless router but getting internet from wifi too?
No, that's sharing from Ethernet to Wifi. I need Wifi to Wifi.
It seems to me that any time your mac could connect to wifi, the other devices could too.
Yeah, I'm trying to think of a reason why he could sign on the Mac but not the other device and I'm not coming up with anything.
As others have pointed out, and I can confirm, it's not possible using only the built-in wifi. You do have some options, however.
1) Connect to the school's wifi using your Mac, then share the connection over ethernet. Plug a wifi access point (or router set to access point mode) into the ethernet port. Connect your DS to that.
2) Buy a USB wifi card. Share wifi from your internal card to your USB card. Connect the DS to the USB card.
EDIT: Note that both of these methods can be detected by your school's IT department, and both are usually against your terms of service. If the school decides to crack down on this, it is entirely possible that you will be caught, and some schools have been known to disable a student's internet access for a semester (or longer) as punishment for TOS violations.