The three-minute long video was shot by a student at Chaiyaphum Pakdi Chumpon School named Peerasak Thaksawirekhaphant who posted it on his Facebook page late Tuesday.
In the video clip, loud applause can be heard while each of more than 20 participants introduces themselves.
Organized for the second time, the pageant for boys identifying themselves as girls was proposed by the students themselves, who got a greenlight from the school's principal and the provincial authority.
A teacher at the school told dpa that the school gave its students an opportunity to express themselves, no matter what their sexual orientation.
"We were not sure at first if any loud activities would be allowed due to the mourning for the late king, but it was approved," Prapasan Poonpayam said.
No student underwent transgender surgery for the parade, Prapasan confirmed.
They still wore school uniforms for boys and were not allowed to grow long hair.
They were apparently wearing wigs during the pageant.
Most online comments on the video were positive, praising the teenagers' confidence and their "fabulous" looks.
The school, which has more than 3,000 male and female secondary students, is located in Chaiyaphum province, 340 kilometres north-east of Bangkok.
Thailand is known internationally for hosting the annual Miss Tiffany's Universe beauty pageant for transgender women, in the city of Pattaya since 1998.