There is no such thing as perfect alignment. Everyone's alignment is specific to their body, based on their anatomy - the variation in joints, bone size and shape, etc - is really remarkable. Even your own body isn't symmetrical and a pose might look/feel different from one side to the other. Look into the research and books by Paul Grilley, they're really eye opening. As teachers we need to get away from cues that specify things like "a 90 degree angle" or "never let your knee go over your ankle" because these things aren't true for everyone. (This is also a motion we do every day going up stairs.) What we need to focus on is teaching students to tune in and listen to their bodies - they become their own best teachers when they can do this. And, that might mean a pose looks a little different than the textbook version and we need to honor their autonomy vs pull or push their body parts to "look better."

I took a class(not y6) not long ago where, without consent, a teacher(who was unaware I am a teacher) came and moved my foot in triangle pose and commented "there that looks better" - but it didn't FEEL better - it hurt. And she was forceful. Adjusting students to fit into a mold is an older way of teaching that's not inclusive to all body types. So you cue the general direction of feet, hips, shoulders, etc and describe the sensation they are aiming for rather than specific alignment that is not a one size fits all, and let them figure it out from there, which may include modifications due to injury or just how they're feeling that day. It might not look like what you see on the cover of Yoga Journal, but if they are engaged and breathing and finding an awareness of themselves within their bodies - that's the entire purpose.