im facing the same problem as i search for a job for this spring/summer. ive looked at a few jobs where they were looking for a chemistry BS with no experience necessary, but they were obviously also looking for a long-term hire. i have been up-front about the fact that i am going to grad school this fall, even though it closes a lot of doors for me.
while it's true that a new hire could leave a company after only 5 months for completely aboveboard reasons, i dont feel it would be ethical to hide my plans from them. i wouldn't judge someone else doing it, but would feel much too dirty about it myself
for your specific situation, unless it really is your intention to bank the couple months' work between now and august, i would tell them about it. you'll know within a month whether you're into grad school or not, and i'm sure they would be willing to wait that long if this isn't just some entry-level position.