Most "networks" were over phone lines those days.
You call in with a modem, and that connects you into a particular computer (or in the general case: a network). BBS for example was just a shared computer on a modem on a well-known publicly posted telephone number that many people called every now and then to check for message.
If you knew the correct telephone numbers and the proper parameters to connect (baud rate, modem type, etc. etc.), you could even get a printer (aka: Fax Machine), a UNIX login prompt, or other equipment inside of an office (and presumably a school).
----
Now why would a school put their grades database on a publicly facing telephone number and hope it doesn't get hacked? Well, that's a good question.
But then again, ATM machines in tiny liquor stores are still largely on this telephone-line / modem technology (I dunno if its still like this today, but even just 10 years ago, a surprising number of ATM machines were still accessible over dial up). So why don't you ask the ATM machine engineers why they think that this practice is safe.
After all, if its safe enough for ATMs, its probably safe enough for a school network. If this thought process is horrifying to you, then welcome to the 80s / 90s era of computer security.
Same folks who built David Lightman's school system, apparently.
Typical computer systems you would dial into would include...
1. Proprietary data services (AOL, Compuserve, etc)
2. BBS systems - typically individual computers running services that let you send messages or files to other users who could then dial in to receive them. Some BBSes were even networked to one another, the largest of such systems being Fidonet
3. Remotely-managed IT equipment - the sort of thing depicted in the movie.
4. Mainframes - universities and large businesses would often have remote access that you'd dial into. This is roughly equivalent to SSHing into an Internet-connected machine today.
5. The Internet - originally only through remote access to mainframes (#4). Later on, data services (#1) started offering open Internet access. (notoriously, AOL utterly demolished USENET's existing cultural norms by doing this) Then companies started just selling dial-in Internet access without other services and this became the dominant use case for modems.
This concept was inverted starting in the late 90s. First, phone companies started offering "digital subscriber lines" (DSL) that provided way more bandwidth to connect to an ISP with. Then, (at least in North America, thanks to various Sega Genesis related reasons) cable companies got in on this and started offering "broadband Internet", too. With the greater bandwidth of these services, it suddenly made sense to send Voice over Internet Protocol (VoIP) instead of Internet Protocol over Voice. So dedicated landline channels became very outdated very quickly, and today we think of voice as just something you send over a multitude of Internet apps.
Also, many schools had internet connections back then. I know our school had a T1, it might have also had a leased line to the state education system for some reason, I would guess the security was very lax back then.
You called a phone number (your modem could do this for you), the target modem answered the call, and they negotiated the maximum (but still ridiculously low) speed. If the target computer needed concurrent connections, you added more phone lines with modems.
But watch the movie anyway, it's a cult classic.
>You called a phone number (your modem could do this for you), the target modem answered the call, and they negotiated the maximum (but still ridiculously low) speed.
Yes exactly what it shows in the movie... and I think he even explains some of that.
Answering a simple question with "see X movie" is the same as answering with "read Y book", and borderline like "do some research" or "google it".