If the teeth are deciduous (baby teeth) then you will be fine, however, if it is permanent ones that are being extracted I would say leave it for at least a week - however, your orthodontist will provide you with expert advice if you ring and ask, or wait and ask at the appointment. Once your permanent teeth are extracted there is a hole in your gum and jawbone where the tooth used to be. This will fill with blood and clot. This blood clot is absolutely essential to the healing process and if it is dislodged (by smoking, exercise, strenuous blowing or sucking motions) then your jawbone will be exposed to the outside world. This is known as "dry socket" and is VERY painful - so you want to avoid this at all costs!