Half an hour ago some had never even picked up a guitar, but two chords later they are playing a song they recognise. Mr Renfrew's enthusiasm is infectious.
Along the corridor another group of youngsters are also learning something new - how to punch each other in a variety of ways. The jab, the upper cut and the round house to be exact, but all without making any physical contact. This is the stage-fighting workshop.
The week-long summer school at Perth Grammar was part of a continuous programme which started in February aimed at boosting involvement in the performing arts. Students from the school and its 12 associated primaries enrolled to take part in a group known as the Perth Young Performers (PYP).
Margo Lennie, education and children's services convener, said:
"Our expressive arts co-ordinator, Edna Auld, had worked with this particular cluster of schools before and thought it was a great place to start. We sent letters out to primary teachers asking them to recommend students who would benefit from the scheme and also had meetings with parents. And then from February there was a weekly workshop after school."
Perth Young Performers is open to pupils from P6 to S2, and helps to ease the transition from primary to secondary. Mr Renfrew says: "By taking part in this programme they know there will be familiar faces when they come here and it makes them more confident."
It also has benefits for other teachers and older pupils. Sixth formers help out with the summer school and pupils who leave the group when they go into S3 are invited to help with activities throughout the year.
Another advantage is that secondary teachers work with a different age group. "The kids get to see us in a different light and we see them in a different way," Mr Renfrew says. "As a secondary teacher it is easy to forget what primary kids are like. We tend to want to wrap them up in cotton wool and forget that they can be tough cookies, so this shows us their capabilities."
The summer school programme, devised by Mrs Auld, Mr Renfrew and Lilla Scott, drama director of PYP, included the core activities of music, dance and drama. New workshops included scriptwriting, street dance and hip-hop, lighting workshops, clowning and physical theatre and film and editing.
More than 70 youngsters took part. Stacey McGregor, aged 14, says: "It's good being around people who like the same things as you do. And because we are the oldest in the group the younger people look up to us and come and ask us questions. And it's nice because you feel like you are helping them and it makes you feel good."
Stacey wants to keep working with the stage management. "I'm going to help with the lighting and directing and you can learn about that on the summer school," she says.
"It's really good fun and you get to meet people from other schools," Samantha Burns, aged 13, says. "I like singing and acting and being part of PYP has given me more confidence."
Mrs Auld hopes to extend the programme to other schools. "There will be about 90 children in the group come September and there is now a waiting list."
* The 12 associated primaries are Auchtergaven, Balhousie, Logiealmond, Luncarty, Methven, Murthly, North Muirton, Pitcairn, Ruthvenfield, St Ninians, Stanley and Tulloch.