Topman suits are a UK brand that is definitely more popular with the younger crowd. Its not a bad choice for a teenager or for a prom suit. They would be tough to pull off in a boardroom or business setting.
Many British celebrities have rolled in a Topman, including David Beckham a few years back. Justin Timberlake has been spotted in them as well. Keep in mind, that they are going to be in custom tailored versions. Even so, many style experts aren't big fans of the lineup, and they think the styles belong in 2005.
They pride themselves on having trendy styles, although they are geared more toward the teenage guy. Think more Jonas than GQ. They also are cut for a slimmer, smaller build, perfect for a guy in high school. Its a better look than a stuffy department store brand for a fun event at school though.