Harrison, six, and Reuben, five, managed to convince Geraint Goodridge wearing Transformer and Woody outfits to their lessons would be completely acceptable.
Their (trusting) dad had picked them up from their mum’s house on Thursday 7 July.
The cunning boys told Goodridge the dress-up day at their school, Glyncorrwg Primary School, was to celebrate the Queen’s birthday.
The two brothers managed to make it all the way to school, giggling in the car, before Goodridge found out he’d been tricked.
“We got to the school and I said to the teacher: ‘It is fancy dress, isn’t it?’ and she said no and then just burst out laughing,” Goodridge told Wales Online.
“They got to stay in their outfits all day.”
Although the dad admitted his kids frequently play pranks on him, he said this one had to be the best yet.
Goodridge’s original tweet was liked nearly 300 times and retweeted more than 130 times within two days of being posted.
Goodridge later uploaded a snap of his two boys holding the front page of a paper with their photo on.
“Mission accomplished hahaha what are they going to do next...” he wrote.