Products like this have nothing harmful. Weight gain for the sake of weight gain is mostly fat. You will need to incorporate this with weight training so that the protein intake becomes muscle rather than just gets stored as fat.
Whatever you eat, it should be in 5-7 separte meals during a day. You burn calories and build muscle on a gentle curve but your food intake is in spikes. Protein, weights, and 6 meals a day and you should put on weight regardless of the caloric source. Some of these meal replacements make it much easier to throw in balanced intake around your traditional 3 meals.
Once you build muscle mass your appetite increases correspondingly to support it. There are some quite inexpensive protein powders you can sub in for your meal replacements that will help retain muscle. Keep in mind at 13 you have one more growth sprut in you so nature isn't done yet. If you were 18 and the size you are that would be more of a worry, but you haven't reachhed your growth limit at at. The weights and eating more meals will simply advance you along the path.
