Every cake I do forms a hard crust around the edge. Most often I can trim it off when leveling, but some times I am unable to. Why is this happening? How do I make it stop????

Generally, from my experience, that is overbaking. What temp are you using? I usually bake 325 degrees and check for doneness by inserting cake tester and not solely relying on the baking instructions. I will bake at 300 for the WASC and turn up to 325 1/2 way.
What recipe are you using? I find the WASC ones are really reliable for instructions and time. Are you baking on the 3rd rack from the top? That is the best. A lot depends on your oven too. Older ones can tend to bake a higher temps.
Also ever since I started using inverted rose nails (ensure to put cake release on them) I have had much more evenly baked cakes. I even use one or 2 in a 11X13. I use more in place of a heater core in a 1/2 sheet and one in a 1/2 ball pan. Cake release is also my 'go to' product (not that it effects the crusting) but makes it much easier to get the cake out and it is flawless! HTH!

I've had that happen when using PAM, or other spray on release products. When I went over to Wilton, or homemade, wipe on cake release, the problem went away.

HTH
Rae

I agree with BlakesCakes..I think Pam kind of fries it around the top edge..sometimes if I wrap them and let them sit for a day or so the top edge softens up though and it is easier to cut off or just leave there if it is softened enough.

I use kitchen scissors and tirim them off

This used to happen to me; then I started baking at a lower temp and it seems to be better. I think my outsides were getting cooked too much and the insides weren't done yet.

I agree that Pam fries the edges of a cake and I refuse to use it.

Baking strips will also help eliminate this, along with a reduced temp. If you toothpick test it, it's ok if there are a few crumbs on the cake .... it's done.

Here's a link to a thread where I explain the science of why the baking strips work: http://cakecentral.com/cake-decorating-ftopicp-6626888.html#6626888

Definitely try baking at a lower temp, wrap with saran wrap afterwards. Also...sometimes a higher sugar content can make the top crusty. If I have recipes that I notice a crispier top, I lower the sugar a bit and find it helps.