I think your thought on the set make up is right on. You might consider adding a 56 degree sand wedge. They will need it as they learn to chip and pitch around the green and they will definitely need it in bunkers. A PW just can’t be used by beginners to hit some of the shots they will need to hit in their development.
As far as the length, we have never recommended or prescribed wrist to floor measurement as a way to calculate the length of a club for a person. We have always used impact labels, stature of the player, physical ability, etc. to fit length. I am assuming, as 8th graders, they are young adults in size. At that age they can still be small or have already had that growth spurt and be adult size. Thanks to your question, I noticed we do not have Ralph’s guide to fitting juniors on the site. I am having that done as I write this, so hopefully by tomorrow it will be available for you to see. It is in the Golf Club Fitting and Performance Book that is currently available. There are charts for boys and girls that can be used as a guide on fitting the length. I would advise using the charts as a guide only, a starting point. As a check on what the charts may indicate, you can have them set up in the address position with a 7 iron and simply observe where the are holding the club and what looks best for them. Mark the shaft at the point they are comfortable and then measure it. You can progress the lengths for the remainder of the set ff of that length.