When it isn't floating, the "Sky High" school is really Oviatt Library at Cal-State University Northridge (CSUN),
which is located at 18111 Nordhoff Street, in Northridge, CA (in the San Fernando Valley.).
The library is located near the center of the CSUN campus.

(The banners hung on the front of the building aren't normally there, of course.)

When the school bus to Sky High comes to the end of this bridge, it crashes through the barricades,
plummets, then soars up into the sky. This location is real, but it's not a bridge. It's the top of the
old Devil's Gate Dam, which can be found west of the west (dead) end of La Canada Verdugo Road,
in Pasadena, CA. (The dam runs parallel to (and just north of) Oak Grove Drive (and the 210 freeway).

The road is no longer open to traffic, but there is no perilous drop-off at the east end (that much was f/x).

In the photo above, the camera is looking northeast.  In the photo below, it's looking northwest.
The trees you see in the background are part of Oak Grove Park (now called "the Hahamongna Watershed".)

The residential neighborhood scenes of "Sky High" were shot in South Pasadena. The scene above was
shot on Meridian Avenue, just south of corner of Prospect Circle at Meridian Ave (looking northwest).

Gwen's house (seen below ), where 'Will' drops her off after her date, then swings on the street lamp
after she kisses him) is just around the corner, at 410 Prospect Circle (in South Pasadena).

[Remember: this is a private home. Do not trespass on their property,
knock on their door, or do anything else do disturb the resident's privacy.]

                      

.

On the other hand, I haven't yet located the main Stronghold house (seen above).
I assume it's in the same South Pasadena neighborhood as the other residential
scenes mentioned above, but I haven't been able to pinpoint it.