• for performance only to other members of the class [and long-suffering staff]
• for acting exercises 
• for text analysis, etc.

you will need to purchase the script, but there is no performance fee.
From the Lazy Bee Scripts buying scripts page, you will need

• "Master Scripts for Copying" or "A Performance set of scripts"
• Classroom performance licence
  [This is free, but applying for the licence tells us how you intend to use the script.]
• Video licences (if you intend to video the show and make copies)

Any performance to people other than class members and staff will be treated in the same way as
a school performance to parents