There are also scripts by writers who have decided to make their work available free of charge – though be aware that just because of script appears online or otherwise for free does not necessarily mean you can print it and the writer may have reserved performance rights – so always check this out before using a script.
You could also consider writing your own script, either as a group using improvisation or as an individual. Shakespeare collections can be picked up cheaply in second hand bookshops and popular plays such as A Midsummer Night’s Dream and many others are available for free as Kindle downloads. This includes many classic plays and though these may be challenging for younger children to perform, many of the plays of Shakespeare, for example, are will within the reach of high school aged kids. It’s not hard to find good free scripts for kids to perform: the first place to look is among the many scripts whose copyright has now lapsed, typically 50 or 70 years after the writer’s death.