If you are looking for an alternative for the static, hand-drawn, poster-board timelines of yesteryear here is an excellent option for your students. TimeGlider
I found that some of the other online timeline creators were not that user-friendly, crashed, or lacked in their capabilities. The free version of TimeGlider is sleek, user-friendly, and has a lot of capabilities. The paid version has even more, but is unnecessary for most purposes.
One of the biggest issues I was having with other web-based timeline software was that they would only allow you to upload photos that were being web-hosted by another program(Flickr, PhotoBucket...) and not from your files on your computer. Since I don't have a web-hosting photo program it made creating a timeline quite difficult. TimeGlider allows you to upload from either a web-host or from your own computer.
The other big benefit is the ease of linking youtube or vimeo videos to your timeline. However it only displays as a link. It would much cooler if you could imbed the video into the timeline.
Another nice feature of TimeGlider is that once you have created the timeline you can share it publicly either in a link, as in the one at the top called Poppy's Timeline, or imbed the timeline into your website or blog as seen at the top. The only real issue I'm having with this is to see the pictures in the embedded timeline the reader has to drag the bottom line, that says "this is the image lane" up