Oftentimes, gifted and twice-exceptional students are starved of their learning, and have to rot in boredom because of it. That's why I'm recommending BBC's schools website to gifted students, because I got most of my information there. There are challenging English, Maths, and Science games for Standard Bitesized, while there are other interesting subjects, such as career management, religious studies, and citizenship (which I read: it's pretty much safe there). One warning, however; one of the games features a Tim Burtonesque character named Mia Cadaver (Cadaver meaning skeleton or skull in Spanish), which could frighten younger children, although she looked cool to me.