Creately has recently improved its interface. It is more intuitive. The brief tutorial upon entering (~4 slides) is enough to get students started with their flow chart.
I like that you can use it without a login to have students quickly show processes in the form of a flowchart. The pricing is not set up for classroom use, so the trial is a better option. If Creately wanted to market this for the classroom, the teacher should have a login and the students should enter a code each time they come to create. Flowcharts would then appear in the teacher acct.
How I Use It
I use Creately with pre-service teachers. They create a flowchart of the decision making process for integration of technology in K-12 education. I then have the teachers explore other options for flowchart creation and compare the user interfaces and cost with K-12 as their target audience.