Starting open source How can I best get students started with an open source project that they would be interested in and welcomed to? I'd prefer that the project is focused on building a piece of software, perhaps a command line based one. My students are high schoolers and are interested mainly in Python, though they are open to exploring other languages. 



(I'm self-teaching, but this is rephrased as if I was teaching a class.)
Since you specifically mention high-schoolers getting started with open source, i have to recommend the Google Code-In. I participated all 4 years as a high-schooler and really learned a lot about open source.

The contest essentially has a dozen or so open-source organizations provide "bite-size" tasks. Each task is mentored, so the students have someone specific to ask questions of. There are both coding tasks and non-coding tasks (documentation, QA, outreach).

The contest itself lasts for only a month and a half (December-January), but the hope is that people get into it and stick around with the open-source projects they contributed to. Many of the grand prize winners do exactly that. See, for example, this blog post by one of the 2014 winners. You can also read my blog post, though i didn't win.

For university students, there is the Google Summer Of Code, which is a similar idea with a rather different format. You must apply to be accepted, and do a single large task, and get paid a stipend.