Well, to be fair, this is a bit of a tough question because it's in question by some of the leading microbiologists right now.
But one argument for this is their ability to reproduce. Note that one of the leading arguments for something to be alive is that they are subject to evolution and hence natural selection: the statements for which argue that there has to be differentiation within a species (meaning there has to be different forms of each virus within a type of virus) and there has to be reproduction-- which is where the problem comes along (because viruses could only reproduce while in a host). So one argument that can be made that they are in-fact alive, is that they reproduce and is subject to evolution.
Do know however that this is HIGHLY controversial-- and if your teacher asks it, it's most likely not a right-or-wrong answer
If he does do it that way, maybe you could show off your brainy-bio knowledge by explaining to him why it shouldn't be so!
