LONDON — At 16, Isabelle Aleksander spends hours writing computer code and plans a career in engineering. Her latest passion is the Raspberry Pi, a low-cost, credit-card-size computer developed to help teach programming.

But when she told her best friend — “he’s male, also into programming” — his response was not what she had expected. “He was like, ‘Wait, how do you know about them? You’re a girl and you shouldn’t be doing that,”’ Ms. Aleksander said incredulously.

She and her friend Honey Ross, 15, are among the few girls at King Alfred School, their private school in North London, with an intense interest in technology. The two, confident and outgoing, say they understand why: computing can seem boring from the outside, populated mainly by nerdy boys.

“It’s sad,” Ms. Ross said, chatting between classes in the computer lab. “It’s such an amazing world. It’s kind of waiting for loads of young girls” to jump in.