How to prevent Batch Create from adding user to main blog?

I'm using Batch Create to set up student blogs in a Multisite install. My ultimate goal is to assign each student user to a role I created with Capability Manager. In order to do this, I have to do two separate imports: the first time with the role "administrator", and then second with the role "studentadmin".

That part is working fine. The problem is that the second import also results in the user being added to the main blog for the site, with that role.

The log shows this:

2013-09-26 02:31:43 [debug] - User bommisi17 already exists (ID: 270)
2013-09-26 02:31:43 [debug] - Blog ( already exists (262), user can be added
2013-09-26 02:31:43 [debug] - User bommisi17 successfully added to blog
2013-09-26 02:31:43 [debug] - There was no explicitly requested blogs; adding user to main blog

I'm not sure, but from what I can tell, this was an issue in core that was fixed in 3.6.1 (which I'm running), so I'm wondering if there is something else going on in the Batch Create plugin.