Eggs are really healthy but depending on what mayonnaise you get it could be an unhealthy sandwich filling. Store bought mayonnaise usually has lots of preservatives or additives that aren't good for you. Whole egg mayonnaise that is home made is actually the best option, healthiest option. Yes, it is high in fat, if you were to put a lot of it in your sandwich. But if you make it with extra virgin olive oil and free range eggs and you only put a small amount of it in then it is absolutely fine!
A healthy sandwich should consist of the following things
1. Wholegrain or wholemeal bread. I know it can be expensive but try to get the good stuff. Some bread companies will claim that their bread is super healthy and full of grains when in actual fact they just put sugar and white flour in it and then chuck in a few grains. Watch out for this because it usually isn't healthy bread. Good wholegrain bread is high in fibre and good nutrients.
2. A healthy spread - This is the juicy part of the sandwich, because lets face it, no one wants to eat a dry sandwich! In your case it was mayonnaise but you could use avocado, low fat ricotta or cream cheese etc.
3. A lean protein - such as chicken breast chopped, lean beef, ham, turkey breast chopped, egg, smoked salmon etc.
4. Vegetable/salad - try to add this in your sandwich as any vegetable has great vitamins in it. You could try lettuce, tomato etc.
Please don't get too caught up about calories or fat. Everyone needs calories for survival, especially when you are at school and still young!
Good luck and have fun!