I thank You for Your extensive answer and sharing Your experiences.
I set up some networks myself, and I do believe that system admin's can do a lot of bullshit. But I think also that there is a difference between being logged in in a greater network and being no subject of special interest. The other case if You are the subject of attention to some people who have admin rights, then you are really f'ckd up. I assume this was the case in Your school. I hope You left this honorable (
For me the first case is more of interest: Not to get in the focus of such attention.
I think the laptop solution has also a disatvantage, because our WLAN connections are protocolled via the MAC Adress of the adapter card. And I think the WLAN encryption is only a matter of interest until the data are deciphered by the WLAN access point. After this point even this packets will transferered in clear Text. At my place this means over at least two switches, a proxy and the router. Enough points to listen. But You are right, that a own laptop will offer more protection.
Thanks for Your advice about the ICMP packets and the firewall, I haven't that in mind.
Infernal greetings, Suboptimo
PS: The THING is called a PCMCIA card.