There is no minimum amount of use that avoids the copyright
laws. If a song is substantially similar to another, and the author of the later work had access to the earlier work, then the use is infringement, absent a license or fair use
. See e.g., Brainard v. Vassar, 625 F. Supp. 2d 608, 616 (2009).
If your use is not for profit, educational and as limited as you suggest, then you won't have any problems, because there aren't any copyright police running around schools trying to catch infringers.
However, if you start selling your arrangement to other schools, etc., then it's not fair use, and you will probably receive a demand to cease and desist and to pay damages to the publisher of McCartney's music
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