Two teens in scout uniforms strum electric guitars, practising for an upcoming concert.
Downstairs, younger children sit cross-legged thwacking away at ukuleles, as a boisterous boy in a Superman outfit bursts into a room filled with modern recording gear.
It is not what you might expect from a slum music school.
But the Khlong Toey Music Program (KTMP) is an oasis in otherwise rough neighbourhood — an estimated 100,000 people densely packed into the heart of Bangkok.
"These are the typical issues from slum communities, such as violence, drug addiction, alcoholism in the family," said Frenchwoman Geraldine "Gigi" Nemrod, who co-founded KTMP.
"Sometimes the family is completely shattered, some of the kids don't know their parents and live with their grandmother."
But every Thursday and Saturday, it's all about the music.
"They can forget about the obstacles of their everyday life and they grow, have fun and express themselves through music and art," Ms Nemrod said.
Nichada Nudang plays lead guitar and is one of the most committed students, although the reason for choosing her instrument is suitably rock 'n' roll.
"I'm a girl and I chose to play guitar to look cool," she said, her grin revealing braces.
"I get to see friends and play music, we feel united together," said bass player "Bank", also known as Suttipong Sawangkhokegruad.
It is the last rehearsal before a big gig — an annual showcase of alternative bands.
"We don't really choose them, if they are able to play the songs we are going to perform, then they're in," Ms Nemrod said.
"The kids are really excited about it."
Life lessons to be learnt
The KTMP began in late 2012, founded by Ms Nemrod and Thai guitarist Siriporn "Amm" Pomwong.
It was inspired by the Playing for Change movement and formally joined that international organisation in 2013.
"We were all musicians and we knew how much music can bring into children's lives, so we wanted to give that chance to other children who didn't have the opportunity to learn," Ms Nemrod said.
The school is funded by donations, with support from the Playing for Change foundation and by fundraising drives like "Pimp My Ukulele", selling ukes painted by Thai celebrities.
Dance lessons are being planned for the future, but KTMP hopes to instil more than just music and movement.
"I think they might learn about perseverance, about commitment, [that] if you want to be good at something, you have to work hard for it," Ms Nemrod said.
"We hope that they will apply these life lessons whenever they will want to realise their dreams in life."
Time to rock
KTMP also aims to build the children's confidence and playing live provides the ultimate musical test.
The Kod Indy Festival brings together more than 100 alternative bands at an alcohol-free event on a sunny afternoon.
Some of the other bands are a lot heavier than KTMP.
On another stage, a drummer leaps off his kit, sending cymbals flying, as the singer rips the strings of the guitarist's instrument.
But none can beat KTMP for sweetness.
After an uncertain start, the band gets going and by the final song of their half-hour set, they have hit a decent groove.
"Freedom and justice … we can make the world a better place," sing the two young girls on vocals, as the uke section sways in time.
Afterwards, guitarist Nichada Nudang is beaming.
"I was shaking onstage, but I felt so happy that people came to listen to us," she said.
"When I did the solo I got a few bits wrong but I did it."
So does she think she looked cool onstage?
"Yeah why not, for me, I felt cool," she said.