Quickest way to level up?

Though i am not at all new to equiverse, i am fairly new to the updated version. I just returned from a 2 year hiatus after playing for nearly 5 years. I read that you can gain exp and level up through showing and riding school, but i was wondering if that was the only way? any tips/tricks to leveling up quickly?

just slightly frustrating coming back to the game and being unable to do many of the things i could once do. 

Hello kami, welcome back!

The quickest way to level up is through your horses. Make a couple or buy some from the RC/other players and enter them into riding schools or show them. With shows you have a lot of options and to be successful in gaining maximum experience you need to have tack and choose classes wisely and the like. I have a guide over in the guides forum called Successful Showing that gives a lot more detail about how to show to win. Technically if you win a lot you'd be leveling up quickest, but it takes some practice, the right horses and some trial and error.
By far an easier way is the Riding schools. Place horses in Schools for up to 10 hours in Group Lessons for 200 EXP per horse. If you make sure your horses are entered into a school of the correct discipline they get a bonus of 75 EXP per horse for a total of 275 EXP per horse. For this reason I recommend getting all horses in the same discipline for easy entering. You can bookmark a Riding school for easy finding, but remember that schools rotate daily so be sure to enter a different school if you want the EXP bonus for matching discipline. (Note you don't get money if you use your own Riding school!)

Another way to rack up the EXP is to train horses, if you don't have an arena and use someone else's it will cost a minimum of $20 per horse going up in minimum cost as the arena grade rises. Training a horse in an arena of the correct discipline gets you a randomized amount of EXP. Training a horse in an arena of the correct grade also give you a randomized amount, so to maximize your EXP gain always train horses in arenas that match their discipline and grade. You get more EXP for horses in higher grades though the price for using outside arenas is higher. (i.e. Novice horse in Novice arena gets 40ish stats, Regional horse gets 120ish stats)

Although a rare random event you can also get 100 EXP while going on Leisure rides and when presented with the bag of money leaving it behind. 
I think that might be it for now, i'm sure there'll be other ways in the future to gain more exp as well.

Awesome, Sabriel. Thank you so much for your help!