This would make a great procedure (Harry Wong). Count to five or hold up one finger at a time when the class needs to refocus. Discuss what occurs for each number at the beginning of the school year and eventually the reaction will be automatic.
One student is selected to be the "Ask Me" person (to help students who need assistance) while the teacher works with small groups-no disruptions for the teacher! Or, the student can put an Ask Me sign on his/her desk