http://www.snazal.com/the-picture-of-dorian-gray-level-4-penguin-readers-simplifie--9781405882293.html?source=webgains&siteid=47684&adnetwork=wg
I even found that there is a comic version, which would be very appropriate and attractive to teenage students:
http://forbiddenplanet.co.uk/blog/2009/the-picture-of-dorian-gray/
http://forbiddenplanet.com/39864-the/
There are also online versions of the novel to work with Bachillerato students: http://www.planetpdf.com/planetpdf/pdfs/free_ebooks/The_Picture_of_Dorian_Gray_NT.pdf
http://www2.hn.psu.edu/faculty/jmanis/oscar-wilde/Dorian-Gray.pdf
http://www.quedelibros.com/libro/74342/The-Picture-of-Dorian-Gray-pdf-ingles.html
Particularly, and knowing the low English level in public schools, I would choose an adapted version of the novel for a class of 1st of Bachiller.
As a pre-treatment activity, and since the film was released some years ago, I would use its trailer:
Or even this video, which before playing the trailer explains briefly the plot of the film:
As a treatment activity, I would ask my students to write down a small student diary, that is, to write down the dates and thoughts they experience while reading the novel, as well as the vocabulary that they couldn't guess from the context.
I would ask them to start writing about what they expect the novel to be, how they think it will continue and whether they are liking the novel or they feel it's too boring. Just a few lines each day they have read something. If students don't feel like writing, they can use vocaroo and record the experiences and unknown vocabulary.
As a post-treatment activity, the students will be asked to work in groups, choose a chapter of the novel and create a photo novel. They will have to take pictures of the different scenes and describe them briefly and creating small dialogues. Also they will have to record them so that at the end we have a picture and recoded version of the novel.