James A. Brooks, in Mark, vol. 23, The New American Commentary, provides additional thoughts:
[T]he trial may have involved illegalities. Illegal trials and perversions of justice have occurred throughout human history in all societies (including ‘Christian’), and this trial may well have been such an instance. No injustice should be excused, but first-century Jews should not be condemned beyond all others for their error.
Brooks also offers the possibility that
what Mark described in chap. 14 was not a formal trial but an informal hearing. Some have compared it to a police interrogation following an arrest or to a grand jury inquiry. Therefore none of the prescriptions of the Mishna [Sanhedrin] would be applicable. According to one explanation of [Mark] 15:1, a formal trial was held the next morning. Therefore this explanation could have some validity, but confidence about it is elusive.
John Grassmick, in The Bible Knowledge Commentary, explains the actions of the Jewish authorities in the following way:
The 71-member Sanhedrin (cf. comments on Mark 8:31), including the presiding high priest, was hastily assembled in an upstairs room (cf. 14:66) for a plenary night session. This was an ‘informal’ trial that required a ‘formal’ ratification after dawn (cf. 15:1) to satisfy strict Jewish legal procedure allowing trials only in the daytime. A quorum consisted of 23 members (Mishnah Sanhedrin 1. 6) but on this occasion the majority were probably there even though it was around 3 a.m. on Nisan 15 (Friday), a feast day.
This hasty night meeting was deemed necessary because: (1) In Jewish criminal law it was customary to hold a trial immediately after arrest. (2) Roman legal trials were usually held shortly after sunrise (cf. 15:1) so the Sanhedrin needed a binding verdict by daybreak in order to get the case to Pilate early. (3) With Jesus finally in custody they did not want to delay proceedings, thereby arousing opposition to His arrest. Actually they had already determined to kill Him (cf. 14:1–2); their only problem was getting evidence that would justify it (cf. v. 55). Perhaps also they wished to have the Romans crucify Jesus to avoid the people’s blaming the Sanhedrin for His death.
Some have questioned the legality of a capital trial on a feast day in light of certain Rabbinic legal ordinances. However, the Rabbis justified the trial and execution of serious offenders on a major feast day. That way, they argued, ‘all the people will hear and be afraid’ (Deut. 17:13; cf. Deut. 21:21; cf. tdnt, s.v. ‘pascha,’ 5:899–900).
In summary, there are serious questions about the applicability of the Mishnah Sanhedrin to the time of Jesus’ trial. Even if the rules from this document did apply, there were numerous extenuating circumstances that could have caused the Jewish Supreme Council to break the rules. Therefore, the fact that some of the procedures called for in this third-century document were not followed does not, in and of itself, cast serious doubt on the historicity of Mark’s account.