Sounds like a fun class project, and really nice for the kids that they can watch these three chicks grow up and share it with their class!
Chickens make amazingly good pets and are a lot of fun, really is hard to believe how smart and personable they are.
With adding more chickens ... With getting more now or waiting, it would especially depend on how many chickens you will have total and if you will be keeping them as long term as pets (not culling them when they get older / stop laying) if you will have a small number you might want to wait until next year at least to spread out when you get them so you don't have all birds the same age but always have a few younger birds that will be doing more laying... chickens will keep laying for years, but their best ages are usually around 1-3 years, the rule of thumb is a chicken will lay about half or less of the eggs at age five years as it did at age one year ... and pullets will usually lay through their first winter but hens will not unless you add light (and even then they will usually molt and stop for awhile).... so over time you would keep geting two or three new chicks when you lose two or three older ones by attrition.
Your girls are old enough where you would probably have to go through the whole introduction thing to add to the flock (with small chicks it is usually pretty easy to just add them, but with older chicks/adults you have pecking order problems) There is a nice article in the Learning Center on integrating flocks you might like to check out, the part about actually combining them is after the quarantine section (depending on where you get new birds, you might need to do a medical quarantine)
https://www.backyardchickens.com/a/adding-to-your-flock and a nice article using the wire method
https://www.backyardchickens.com/a/introducing-new-chickens-using-the-see-but-don-t-touch-method. If you get different age birds, generally it is best to wait until the birds are about the same size, and then have a long period of the two flocks being able to see each other but no touching, through wire seem to work best, ie dividing the coop into two sections or keeping the new/younger ones in a cage inside the coup for a couple of weeks to a month at least. The chickens will get to know each other and sort of work out a pecking order before actually coming in contact with each other. After a week or two, letting them free range together is a good idea and should help... It will take a couple of weeks to get the pecking order sorted out.
With Silkies, they do need special care, they tend to get picked on by other birds, and there is a good chance you would need to keep them separate from the general flock.
With finding birds, you might try your state thread (maybe you can find more school hatches etc, or put your name in for the next set of chicks if they are going to do it again etc?) .... with actual "rescue" birds be very very careful (read the quarantine section in the Learning Center) it is all too easy to bring something contagious home to your flock.... also most hens that people will be trying to place / give away will be spent / old hens that will not do much laying (most people who breed for egg production will cull hens when they go into their second or third molt because their egg production will go down because of age and so they don't have to feed a non-laying hen over the winter)... you usually see lots of these at sales or on Craigslist etc, or even the battery hen idea where the big producers will sell/give them away.