I'm not making excuses, but she is probably engaged on missions -
Collecting resources
Organizing future trips and speakers
Running risk assessments
Filling in reports and statistics asked for by line manager
Chasing reports and statistics from junior staff for whom she has line management responsibility
Checking stock
Photocopying
Replacing photocopier ink cartridges
Attempting to mend photocopier that previous user jammed up with paper
Putting up new displays
Taking down old displays
Making phone calls
Having whole staff meetings
Senior management meetings
Year group meetings
Faculty meetings
1 to 1 meetings
NQT support meetings
Appraisal meetings
Personal development meetings
Writing merit certificates
Filling in book of who has had/not had merit certificate this year
Checking supply teachers are in the correct rooms and are organized
Sorting them out when they are not
Searching for lost items
Reclaiming borrowed items from other staff
Reading documents so she can keep up with latest directive
Listening to complaints from lollipop man and trying to identify the 'culprit' from his description
Thinking about whether she will have time to visit the toilet that day because she has morning assembly duty as well as playground duty as well as lunchtime training tutorial on new school software as well as after school club to run
And therefore she spots you and is terrified that you might want a lengthy conversation with her and interrupt those things.
It might even be a conversation where you give her a job to do! "Oh by the way Mrs Smith, please could you watch Tabitha and Grace's tap dance and help them synchronize a bit better as it's the Brownies' show this weekend?"
Hence, it's rude, but life has taught her to pretend she hasn't seen you.