The Criminal Court ruled to assess the matters – listed on two writs – in one trial.
The trial will be separate from a judicial hearing for six leaders involved in the same protests.
Prosecutors have decided to try organisers of the rally in two batches based their alleged individual roles in two incidents – trespassing at Government House and blockading Parliament.
In the first batch, the lead defendants include Sondhi Limthongkul and Chamlong Srimuang. Their trial began on December 27.
In the second batch, Somsak Kosaisuk and Suriyasai Katasila are among the defendants.
The 15 yellow-shirts face key charges of property destruction, unlawful public assembly and inciting people to break the law. Of the 15, Somsak and Suriyasai face additional charges for trespassing at Government House, while the remaining 13 must answer to charges that stem from the blockade of Parliament.
After the charges were read out, the 15 pleaded not guilty.
The court approved bail, or temporary release, for the defendants pending the outcome of the judicial hearing.