Back and behind

I also teach ESL (as a volunteer) and if you have 3 people of about the same level, you are way ahead of my unbelievably mixed level group of senior citizens!!

Well, as nouns your back runs from shoulders down to waist and your behind is your bum/tushy/butt/pick a better noun. I assume that's not the question you are asking?

As prepositions, behind and "in back of" are probably almost synonymous. He is behind me in line. He is in back of me in line. That one works. In back of is more immediate however. He is behind me in line can mean anywhere in the line. In back of me in line usually means only one or 2 back. Note, he is 2 back, not 2 behind. So if we were both walking through a museum and I got outside first, I might say to someone my husband fell behind me in the museum, that is, he is lagging. Also the figurative is behind. Nicklaus is behind Woods in the British Open standings, not in back of.

Wow, way more than you wanted, huh?

The difference in many instances is just how we use it. In my opinion the only way for people to learn this kind of thing is listening and practice using it. Depending on their native language there may not even be a way to translate it in a way that makes sense. There's really no way to explain why something is 'back there' and something else is 'behind there'. If there is then it's probably too complicated for them to understand, I suggest moving on and hopefully they'll get it later.

Back is more of the physical place, whereas behind is more the relationship between two things. If someone creeps up on you they are standing in a place called "back" but their spacial relationship is "behind"...I think...