hey, love galinda and am playing her in a school musical.
how high does she have to sing?
In the Broadway musical's sheet music score, her highest note is a double high C or a C6. There are several optional notes that can be sung in the high soprano numbers such as Thank Goodness and No One Mourns the Wicked.
The professional shows usually cast someone who can hit the optional high notes as a bonus, while some stick to what is written. There are some cases in which the Glinda actress does not hit the note at all! Alli Mauzey does not hit the last note in the Finale, choosing to play the part of a girl who lost her best friend and stays silent.
As for the acting, the part of Glinda allows many squeals, screams, and other girly noises that is part of her improvisation. Kristen Chenoweth does the squeal in Popular. Some standbys do a little trill in Dancing Through Life. Experiment and see what works best for you.
I'm sure the highest note you will go to (optionally) will be a double high G, or G6.
Break a leg!