TEACH them how to determine 1/4" seam without just relying on the foot. They need to learn how to true up the 1/4" seam....the first year I learned to piece I just trusted the markngs on my nachine....my blocks were irregular. Once I did the strip test for determining seam allownce and I adjusted my needle postion...much better outcome! Eleanor Burns demonstrates this on her website.

Sandy

I think one of the most important things in keeping a true 1/4 inch seam is to be sure the seam is properly set together as it starts into the machine straight and is held in place so that it exits the machine straight. I have observed that not keeping the pieces straight behind the machine pulls them out of line and makes the seam inconsistent. So, whether you use painters tape, a sticky pad, index card or moleskin tape, be sure it extends beyond the feed dogs.

In addition to the info already posted, sitting directly in front of the needle helps to maintain straight seams.

Don't forget that not all machines can have needle position adjusted.

And until you're proficient in accuracy, go slow...there's no need to "push the pedal to the metal".

Thanks to all for the suggestions. I can use them too. For some reason (age??) I am not getting consistent sewing and my cooking is terrible, all my teeth hurt, and I'm gaining a lot of weight. Anything sound familiar?

Yes. All of it.
I think something with a ridge works best. A couple strips of tape. Old digital hotel key or non identifying strip of credit card plastic. Stack of sticky notes. The guide should start at least an inch before the foot, and also extend towards the back. They should also have the pieces lined up before they bring them to the machine to be sewn. Some students may benefit from using a stiletto, especially at the beginning and ends of the seams. Inexpensive bamboo skewers are good.

I have a sheet of craft foam from the dollar store (the stuff is either in sheets or cut into shapes). I cut a nice straight edge piece with my rotary cutter and ruler and tape it to my machine with painter's tape. It makes a little "wall" to keep the edge of your fabric in place. Like Jan has a pic of in her post. Just instead of a pad of sticky notes, I use that craft foam instead. It's cheap, I can replace it easily, it's flexible and I can cut it to any size I want.

I like to use the command adhesive with velcro on the back It works very well and no sticky residue on the bed of the machine

I use the Command strips. I buy the large strip refill pack. I don't take the paper off the top side so thread and lint don't get caught up in it. The most accurate way to test for 1/4 seams is to sew six six inch long strips together and then check the size. You can tell immediately if you need to adjust the seam to be narrow or wider to get the correct width. The width of the seam doesn't matter as much as ending up with the correct finished size.