|Unless someone here has "hands-on" experience with those particular appliances, and I do not, they won't be able to tell you what you wish to know.|
What I can tell you is, if it were me researching this, I would carefully read the spec's on each device and do my own calculations. I would also read any/all reviews I could find on the devices I was looking at.
Why are you using wireless extenders and not Access Points? Are the devices you linked above less expensive than AP's? If not, I suspect you'd be able to find AP's that are more cost effective and have the same capabilities.
If you're not going to use the managed aspect of the switch, buy one that isn't managed. It would cost you less money.
You mentioned 17 clients per AP but I'm wondering how you plan on evenly distributing them?
Unless you use MAC mapping to the AP's, clients will always connect to the closest device. Left to that, you would end up with overloaded AP's and some that are basically unused. MAC mapping devices to AP's would require a lot of admin time.
If it were me, I'd use a wired solution since students will be sitting in a classroom (I assume) and not roaming around. Wired is infinitely more reliable and a whole lot less hassle in the long run. Setup the same type of "guest" access network for students (ie: they only have internet access, no access to internal resources) and put a DHCP server on that network and then all students have to do is plug in.
You don't need to spend money on wireless equipment either this way.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.