I usually don’t post much about web applications that require the use of a webcam just because webcams are problematic for school computers for safety issues, along with needing to dowload its required software.
However, if you can use a webcam, Send Shots has got to be just about the easiest way to send a video message to someone. No registration or download is required — just record and send. There are no ads, and there’s no way to access other people’s video messages. You can post the url address of the video on a teacher or student’s website.