When I was in High school (in the early 2010's), I certainly didn't have room for a laptop or tablet on my desk! Just a notebook or binder would fill most of the available area, with room for an eraser and spare pen or pencil along the top edge. My calculator would sit on either the left or right page of my notebook, whichever I wasn't writing on at the moment. The textbook would be open in my lap.

The only way a laptop or tablet would have fit would have been to either have my notebook over the keyboard and touchpad, do away with the notebook and do all work digitally, or do away with the physical textbook and put it on the laptop. My experience with e-learning and e-textbook platforms suggests that I very strongly recommend against the latter two options.

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Off the top of my head, a few more reasons:

Although TI's calculators are expensive ($100-$150 each, depending on the model), they are cheaper than almost all devices that can run Matlab, Octave, etc. The dirt-cheapest of Android devices are cheaper to buy, but have higher administrative requirements, are probably too slow to run CAS well anyway.

Calculators are much sturdier than smartphones, tablets, or laptops. Frangible case, thick ABS shells, rounded edges, low-tech big-pixel LCD. Calculators are subject to, and withstand, a great deal of use, neglect, and outright vandalism at the hands of teenagers. Tossed in backpacks, dropped onto floors or sidewalks, sharpied, spray-painted, engraved with anything available... The calculators survive this environment pretty well; it is entirely possible and reasonable for a student to be given and use a ten or even twenty-year-old calculator in class.

Not only physically enduring, the calculators remain relevant for long periods of time. Even though TI has released new models since the TI-83, many classes will allow any calculator in the family. Some students will have brand-new instances of the latest model, some will have hand-me-down or secondhand calculators, and other students will use loaner calculators from a fleet that the school has retained for years. Maintaining such a fleet is far cheaper and easier than maintaining a fleet of up-to-date, secure, and working laptops or tablets.

Calculators require less charging than laptops or tablets. A cheap quartet of AAA batteries can last a semester, or even a whole school year, in a calculator. If the batteries do die at a bad time, teachers will often have a stash of extras that can be swapped in by the student, allowing them to finish the class or exam. Tablets or laptops require banks of chargers, usually on mobile carts, and all of the cables need to be properly plugged in every afternoon, and sometimes before then. Any failure will result in a student who has to switch to an entirely new device (if one is available at all) in order to keep working.

Calculators are instantaneous-on. You don't need to wait agonizing minutes of valuable class time for the OS to boot, the network to connect, typing a username/password, loading the desktop, and launching a CAS program. At best, that process could take only a few seconds. At worst, it could take many minutes. Anecdote: halfway through one of my semesters of high school, some of the school computers (including one I was assigned to) were updated from Windows XP to Windows 7. The login system recreated your profile from scratch at every login. The result was that it took about ten minutes every day just for me and the 6 or 7 other guinea-pig students to log in to our computers -- a big impediment in a computer graphics class, in a school where classes were only 47 minutes long.

Calculators always work, and they always work the same way. They don't have mandatory security updates, certificate expirations, obsolete network configurations, missing user profiles, misconfigured product key servers, surprise UI updates, cloud-only functionality, or anything else like that. If it turns on, it works, and is exactly the same as it always has been. Another anecdote: In college, my ios app class was thrown in to disarray when we came in and found out that the university had updated to the latest version of xcode. The new IDE also came with a new version of swift. Our project was due later that week, and none of our code would compile. The instructors had to push the deadline back and create a "what we told you last week is different now" lecture the next day.

Calculators are much less of a classroom distraction. Even though they can have third-party applications and games installed on them, they have an overridingly clear purpose: to perform calculations. Doing anything else on a graphing calculator is tedious and difficult. Calculators can only communicate by wire, have no sound, and have smaller (and usually un-backlit black-and-white) displays pointing straight up from the desk, so they can't really engage/distract more than one student. Contrast with laptops or tablets, which students know are able to browse the internet, watch videos, and engage socially with their peers. Even though the devices can be "secured", the value of workarounds is much greater than for calculators. Savy students will spend more effort developing workarounds for internet-capable devices, and their peers will jump through more hoops to perform them. When workarounds are in place, their use can also be much more distracting than a calculator: students covertly chatting in class, playing flashy games, or having sound play out the speakers.