In Sandquist v. Lebo Automotive, Inc., ___ Cal.4th ___ (Jul. 28, 2016), the Supreme Court considered who decides whether an arbitration agreement prohibits or allows class arbitration: the arbitrator, or the court? Justice Werdegar's majority opinion holds that the answer depends on the parties' agreement, which is construed under state-law contract principles. Slip op. at 4-5. In the particular arbitration clause before the Court, the parties agreed that the arbitrator would decide. Id. at 6-12. Federal law did not mandate a different result. Id. at 15-29.
Justice Kruger filed a dissenting opinion, in which she was joined by Justices Chin and Corrigan. The dissent reasoned that federal law does, in fact, dictate that the class arbitrability question be determined by the court, not the arbitrator, because it is a "gateway question of arbitrability." Dissent, slip op. at 2.
A cert. petition seems likely here.