If your recorder is a normal descant recorder(usually use in Primary school), A and B are not really high note since that recorder can play up to C and D.
When you play these notes, you need to close half thumb (left hand)? If you just cover half of the hole, you cannot play that high note. MOST IMPORTANT is that you need to slide your NAIL into the thumb hole and make the hole as small as possible, leaving only a little slit.
This must help.
If you need to play even higher note, you need to turn your thumb to a particular angle. That angle is different from different recorder and different person.
2008-04-15 09:28:29 補充:
1. When you paly high notes, you should blow a LITTLE bit harder than low notes (Use warm air for low notes).
2. Don't just cover your thumb hold small, you should slild your nail into the hole and the hold should be smaller as you play higher notes.