Attack the net. Add this to your list of tactics the average high school girl doesn't go up against very often. If you're at all comfortable at the net, try to get there against the type of player who loves to hit 3-4 feet over the net, medium paced, right down the middle. You'll take her favorite shot away from her, and what she tries as an alternative might not work for her at all.
Jam her two-hander. A two-handed stroke is especially hard to execute when the ball is going right into the body. In baseline rallies, you won't get many chances to "jam" your opponent, because she'll have time to move, but if she stands in closer to return serve, it's worth a try. If, at the net, she uses two hands for her backhand volley, aim a little above her right hip. With little time to move and no one-handed backhand volley to save her, she'll be in trouble.
Send her a high, deep lob. Yet another shot that works well because of its rarity, the high, deep lob can simply confuse your opponent. She'll wonder whether to try an overhead after the bounce, which she's likely to miss due to lack of practice, or to let the ball fall and try a groundstroke from way back. Test her, especially in a situation where she's under pressure.
Pull her into "No Person's Land." Your opponent should know better than to get caught in this area between the baseline and the service line, but that doesn't mean she won't do it. Hit a ball 10 feet inside her baseline, then if she doesn't get back behind her baseline in time, simply hit the next ball so that it lands deeper than she's standing and away from her to her right or left. It's an easy point for you, and one you might be able to re-create many more times. If she starts to catch on and scramble back quickly, try a drop shot just to change things up.
Some of the suggestions here might be more difficult than others for you to try. After all, if your opponent is a 3.0 - 3.5 player, so, most likely, are you. You're almost sure to find some of the suggested shots easy, though, and the fact that you're playing creatively and probing your opponent's weaknesses will be a huge advantage in itself. It will also make your matches more fun!
