There's a meme going around among piano teachers which shows a cat with an extremely surprised face, and the caption indicates that it is the teacher's reaction when the student is playing something in the key of A Major and "actually remembers the G sharp!"
It's a big hit with teachers, of course, because teachers like knowing that other teachers are just as frustrated as they are about the same issues. And the cat is really cute.
That isn't going to stop your student from continuing to abuse the G natural, though, and you might be wondering if there is a better way to go through life than to pleasantly remind them every time they do that. I thought I'd offer a few observations. The first is that the primary way I was taught to do this, by playing scales, is largely a waste of time.
Memorizing key signatures often seems irrelevant to the student, much like asking what happened in 1858. Scales can be the muscular equivalent of that. If you are going to have the student start every lesson with scales, which students almost universally hate, why not try something different?
Your approach can depend largely on the personality of the student: a few times I have actually taught all of the scales in one lesson, rather than parceling them out a week at a time and trusting the student to remember what A Major is supposed to feel like when it is needed. In these cases I go all the way around the circle of fifths and have the student play each scale while explaining how the system works. And the students actually enjoyed it. In fact, they had fun! This avoids the problem of parceling things out a bit at a time and making scales into a thing that you just have to do at the start of every practice, which have nothing to do with the music you want to play, and are an inviolable pattern of boring notes.
Understanding the entire system of keys is something you can try (mainly with older students, I think)--not to mention that it will seem like a challenge to do them all at once, and that can be exciting!, but if you are stuck on one scale a week, then don't let that scale remain an unthinking up-to-the top down-to-the-bottom routine. Change up the fingerings. Have the student try 1 2 3 2 3 4 3 4 5 4 5 6, etc. or 1 2 3 4 2 3 4 5 3 4 5 6 and so on. If you have an engaging personality you can get lots of things to sound fun that aren't if you don't. The idea here is that the student has to learn to think in A major rather than just put it on auto-pilot and cruise up an down in a familiar pattern that, even if mastered, does not guarantee that G sharp is going to seem a preferable alternative to g natural in measure 7 of their new piece, in the right hand. There needs to be a connection.
And here's where it gets weird. The one thing that has helped me the most, I think, has nothing to do with scales. I learned to improvise. Make up my own tunes. Quite a useful skill when you have a deadline and no time to practice, or suddenly have to fill time with music at a party or a church service that you didn't know about beforehand. If you have to create something in A Major, you think about it more. Have the student make up melodies using A major. The G# has to be reinforced every time you need it, randomly, in the wild, on demand, and while thinking about other things (like how I want to melody to go) rather than as a thing that happens near the top of a pattern I don't want to play.
As always, the keys are to make one have to think about it--often, and to reinforce the idea--often, rather than the make that G sharp something that exists out there in the ether that I have to do because teacher reminds me to do it once every six weeks when I have a piece with a G-sharp in it. Then I don't remember because: who needs to know? If it's part of a system I understand, it it is a challenge I like to undertake, if it is a pattern I use frequently, if it is just plain fun because I like the feel of a raised fourth finger, things are quite different. Ultimately success motivates and carries the rest of it along.
Of course, if the student never sees a piano between lessons this will be less effective. Eventually you should make room in your studio for somebody who does notice an instrument once in a while.
But in the meantime, give them a reason to know their g-sharps. Eventually it will seem natural. Pardon the pun.
Now get out there and look sharp!