I have come in from several professions, so I was 45 when I began my teaching career. And I thought that I knew pretty well where I was going. However, I didn't realize I was a closet idealist, and nobody told me that someone could fail. And it was devastating to me to realize that I had students that were failing, who weren't passing despite everything I could do. It was heartrending to discover this.
And I would like to tell the people coming in, "Yes, keep your standards high, don't lower standards." Under no circumstance lower your standards for these students—expect the best, expect the top. But remember that there are those who, for whatever reason, are going to fall through the cracks. You can't save them all, but that doesn't keep you from trying your heart out.