Viaggiatore is right. Even though "aperte" looks like a past participle, it is actually an adjective in this context, meaning "open" (although translating it as "opened" -as an adjective- may also be acceptable). The reflexive form is used for "the windows were opened/ have been opened". This link may help (see "anticausative"): http://www.yearlyglot.com/what-the-heck-is-a-reflexive-verb/
That would be "Le finestre si sono aperte." (The windows have been opened.) http://tutorino.ca/grammatica/2007/7/17/the-italian-passive-voice-la-voce-passiva.html
Please note that participio passato of aprire is avere aperto, so "the windows are opened" will be translated as "le finestre hanno aperto". As with many verbs, the adjective is similar or same as the participio, so aperto means "open", thus la finestra e' aperta, and in plural "le finestre sono aperte"
There is another way to look at this (through an open window). The perfect tense means it is completed action in past time. Quindi, if the window were opened (in the perfect) they would in the present be open. Hence the translation the window are open (because they were opened perfectly in the past). Ha ha.. Le finestre erano aperte would translate "were open" but they may not be open still...
I guess that's one way of looking at it. Actually, this is the present tense, since it's talking about the state of the windows now. It's the present passive. The present perfect would be "the windows have been opened" and the simple past "the windows were opened", so I guess the only reason for these passive sentences being here is either that the writers of the course don't really know grammar that well or that they are trying to keep us on our toes by teaching us something different in this lesson.