Why WEP wifi network is not found by SK17i?

I cannot to connect to a school wifi network. When I go to wi-fi settings the phone shows all the unencrypted networks and the WPA encrypted networks. But this network is encrypted with WEP. Any clue where's the problem? All the issues I googled are that the network is visible, but the connection fails. In my case the network is NOT visible. From my PC the connection to that network works fine. I can NOT access the configuration of the router. (E.g.: Change the encryption to WPA.) My phone is Sony Ericsson Xperia Mini Pro (SK17i). Android version 2.3.4.