Not the point I know, but from what I have read that translation seems pretty suspect. And even if it were accurate, I'm not sure why it's particularly relevant.
As for your question about the translation, I am not an expert either, but there is an entire section of the relevant Wikipedia article about the etymology of the name:
"In the town of Maiduguri, where the group was formed, the residents dubbed it Boko Haram. The term "Boko Haram" comes from the Hausa word boko figuratively meaning "western education" (often said to be literally "alphabet", from English "book", but the Hausa expert Paul Newman says it derives from a Hausa word with meanings such as "fraud" and "inauthenticity")[35][36] and the Arabic word haram figuratively meaning "sin" (literally, "forbidden").[37][38][39][40] Loosely translated, the name could mean "western education is sinful", which might symbolizes its strong opposition to anything Western, which it sees as corrupting Muslims.[41] However, this interpretation of the name is disputed.[42] Locals who speak the Hausa language are also unsure what it actually means.[43]"
It doesn't answer why they only targeted girls, though. Unless it was a girls-only school.
EDIT: To be specific: The name “Western education is sinful.” says nothing about why they only targeted girls. This group's history and background might suggest their motivation, but here, in this subthread, only the exact name (or rather, translation) was under question as a possible explanation for their motive.
http://www.csmonitor.com/World/Security-Watch/Backchannels/2...
Article: I mean, it actually pretty much does... but umm... language is complicated?