It would depend on the browser. But most likely if it is Firefox (or Mozilla I guess), which can be set through their profile file.
When first created, a user's home directory is based on the skeleton setup in /etc/skel/ (by default). This includes all hidden files, basic directories, etc.
You could copy over a Firefox settings (with Yahoo as the homepage) folder (should be .mozilla in your home directory) to the skel folder. Then maybe change the write permissions to root (otherwise they could change their homepage back).
Hopefully that was clear-ish...