What the heck. My tongue never goes anywhere close to my teeth when I do an S. It's basically on my palate, behind my upper front teeth. If I put my tongue between my teeth, if it's too far back, it's "sh". If it touches my top teeth, it sounds like a softened "th". If it's too much towards the front, there's no sound but saliva drops do come out.
Now if I try to put my tongue BELOW my lower teeth, all I can do is a very windy "sh" or an "h". All of the sounds I have made following (and obviously misunderstanding) your instructions sounded undeniably and deeply retarded for someone to use as an S sound. Like someone with partial paralysis.
I really think I'm not understanding. To make an S, the tongue's tip just needs to touch the palate. I can't imagine it otherwise. So what the hell is it doing on the front teeth, and below the bottom ones at that?