He taught at a secondary school in the eastern part of Singapore.
Unknown to the female students in the school, the teacher was busy pointing his video camera lens up their skirts as they walked up stairs or sat at the canteen.
The video recordings were later found on his mobile phone.
On Wednesday, Seow Swee Hong, 39, pleaded guilty to multiple offences of insulting the modesty of women, Channel NewsAsia reported.
There were 10 charges and another 84 were taken into consideration by the prosecution. At least half of the offences were committed while he was on police bail.
He was caught on 25 July 2011 when he was at the Popular bookstore next to Toa Payoh bus interchange.
A store manager at the bookstore saw him taking videos up the skirts of female students there.
At one point, he knelt down to position his mobile phone near one student's legs to shoot her underwear and thighs. He continued to follow her and took more recordings as she walked or stood in the store.
Earlier that day, the secondary school teacher had also taken similar video clips of some female students in the school, court documents showed. There were also recordings taken at supermarkets on different days.
Most of his victims were girls dressed in school uniforms.
Seow’s lawyer, Mr David Liew, said his client had obsessive-compulsive disorder and could not control himself.
The sentencing is expected to take place on 12 October, Channel NewsAsia reported.
If convicted, Seow could be jailed for a year and fined on each charge.