Reddit mentions of Mechanical Numeric Keypad GATERON Brown Switch 21 Keys Mini Numpad Wired Ice Blue Backlit Gaming Keypad Portable Keypad Extended Layout White Magicforce by Qisan

Every number pad has arrow keys and Del, just turn off number lock.

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https://www.amazon.com/Velocifire-NK01-Wireless-Mechanical-Keyboards/dp/B071VBVYPR/ref=sr_1_5?keywords=mechanical+number+pad&qid=1557931863&s=electronics&sr=1-5 or

https://www.amazon.com/Mechanical-Portable-Extended-Magicforce-Qisan/dp/B073WB83RQ/ref=sr_1_9?keywords=mechanical+number+pad&qid=1557931863&s=electronics&sr=1-9

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or if you want everything, going to have to accept some crappy membrane: https://www.amazon.com/Bluetooth-Aluminum-Rechargeable-Wireless-Compatible/dp/B07L139XXZ/ref=sr_1_9?keywords=number+pad&qid=1557931742&s=electronics&sr=1-9

Oh man, yea that would definitely get annoying having to program every single key one at a time, something like QMK would be way more powerful and faster to use whenever you want to switch your layout since you just need to edit your layout in a text editor, turn it into a hex file and then flash it.

I got this for my numberpad, I know there are definitely better ones out there, but honestly this one does the job fine

There's the Jelly Comb one already mentioned in the comments if you're after a cheap one, it's the one I personally have. Downside is that it doesn't use standard numlock size keys (note the top row and the plus button. Comes in black or white (black one is most common), with Outemu Blue switches.

Another cheap option is the Velocifire numpad, it's wireless (2.4 ghz USB dongle) with Outemu Blue switches. It has a 4 1x1 key top row and only comes in black.

For something a bit nicer there's the Ducky Pocket, comes in various Cherry MX switches, it's wired and has a LED display, RGB backlighting, doubleshot PBT keys. It only comes in black and has a top row of 4 1x1 size keys (not sure which row, probably R4?).

Personally though I'd advise against going with the first two options as Outemu Blue switches are loud (same with any clicky switch) which wouldn't go over too well in a classroom environment.

edit: I just noticed that the Qisan (Magicforce) numpad is in stock on Amazon: https://www.amazon.com/Mechanical-Portable-Extended-Magicforce-Qisan/dp/B073WB83RQ/
https://www.amazon.com/Mechanical-Backlight-Portable-Magicforce-Qisan/dp/B01MZ4Z2DK/

Gateron Browns, comes in white or black.

yeah it is pretty ridiculous how much choice there is with mechanical keyboards 😂

the tricky thing about switches is that it's kind of a myth that one switch is better than another for typing or gaming. ultimately, personal preference and what you like will be more important than whatever is on a spec sheet. if there's an electronics store near you that has mechanical keyboards on display, i would strongly recommend heading over there and trying some out before buying, as mechanical keyboards can feel quite different than regular rubber dome keyboards. even if they only have gaming brands available, it would still give you a good indication of what you're in for if you do buy a mechanical keyboard.

i'd say the ducky one 2 mini would be a great choice since you could always just get an external keypad, some of which would offer more functionality than a regular keypad anyway.

i'll link one external keypad that i personally like down below, as well as some other full-size options that you could consider (none of them are white unfortunately):

external keypad: https://www.amazon.com/Mechanical-Portable-Extended-Magicforce-Qisan/dp/B073WB83RQ/ref=sr_1_50?keywords=keypad+mechanical&qid=1563299944&s=gateway&sr=8-50

ducky shine 7: https://mechanicalkeyboards.com/shop/index.php?l=product_detail&p=4333

ikbc mf108: https://www.amazon.com/MF108-Mechanical-Keyboard-Aluminum-Keycaps/dp/B06W5SBQNG/ref=sr_1_2?keywords=ikbc%2Bf108&qid=1563293226&s=gateway&sr=8-2&th=1