Hey I start jump training all my potential eventsrs early, but not by jumping. I do lots out on a trail just stepping over logs or going through puddles, as a horse doesn't know the differance between a puddle/log/ditch out riding compared to what they see on xcountry.
For weak unbalanced horses I do lots of hill work. At least a basic flat work on a hillside for 30mins once a week. Really helps them strengthen and give them variety other than arena work. I've strengthened lots of weak thourogbreds up this way.
Like said above Cavaletti, poles etc can help but you probably don't need to specifically jump school yet