Can you say "yep" or "yeah" instead? This was always easier than "here" when I'd block in school. 'ere or year (but make the "y" sound silent so it sounds like 'ere) are also tricks that can work. Also if I tell people I have a stutter, I find I'd block in front of them way less. Try and tell your teacher in private then you may feel less anxious when they call your name.