Yes, practice can increase scores, but it is unethical to give a child either the CogAT or ITBS to practice for another administration.

That said, I administered the ITBS in our homeschool yearly, and over time my kids got really good at taking that sort of test. I also administered the CogAT on a few occasions, but I timed it so that it would be a year from when they would take it for GT eligibility.