Videconferences can be used for online meetings at a distance at a set time for a selected group of people. Some of the videoconferencing tools you may find useful in your classroom are:
Flashmeeting (https://fm.ea-tel.eu/fm)
OpenMeetings (https://code.google.com/p/openmeetings)
BigBlueButton (https://bigbluebutton.org)
Skype (https://www.skype.com/en/)
WiZiQ (https://www.wiziq.com)
Google Hangouts (https://www.google.com/+/learnmore/hangouts)