User blog comment:DiegoAnime3000/Blake and Tukson/@comment-7387716-20140727104420

And maybe Emerald and Mercury would be going after Blake next, similar to what Tukson has done in leaving the White Fang. Hence the reason why they are at Beacon and hanging out with Ruby and her pals. Probably the most plausible reason I have for those two to be at Beacon.

Or Cinder wants to get revenge from the fight in the first episode and have Mercury and Emerald sneak in to Beacon and find out more about Ruby and possibly kill her, if they are taking extremes in their plans.