2. Tell your colleagues when you're ready for the kids to know. SOMEONE will let it slip at some point. If you submit a letter of resignation, tell your colleagues before then - a school board meeting isn't a secret, and resignations go to the Board.
3. You generally can't. Take your days or see if your district will let you donate them to a pool/bank. Otherwise, all you can do is use them - combined with a pile of money - to purchase another year of service.
Keep your state days if you plan on returning to a classroom or administrative role, however - they stay with you. Local days go "poof" when you resign. Some districts offer to pay for them, though.