Would this would be good for a 2nd grader with child size desks at school?

Yes. It is the right size for my (small, but long legged) 9 y.o. (Fourth grader)

I have four of these and one ball has a leak where can I get replacement balls?

They offer replacement balls on Amazon: https://www.amazon.com/gp/product/B073XZPZYC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Can the castor wheels be replaced with the roller blade style wheels that come on some desk chairs now? Is their attachment point the standard size?

I’m not sure what the standard size is but hopefully this helps

Would a 3 year old be able to sit on this chair comfortably?

My son is 47" and it's perfect for him...he's kindergarten. However, you need a very low desk. We have it paired with a lifetime adjustable plastic table on the lowest leg height. It's the rectangle portable table from Costco. I would say it is going to be too small for him in a couple years.

Assembly Required	Yes
Shape	長方形
Item Depth	21 inches
Item dimensions L x W x H	86.4 x 45.7 x 53.3 cm
Product Care Instructions	乾拭き
Global Trade Identification Number	00018713622419
Material	金属
Manufacturer	Gaiam
Special Feature	サイドポケット, ロック
UPC	018713622419
Model Number	05-62241
Sport Type	ヨガ
Batteries Included	No
Maximum Weight Recommendation	79.38 kg
Product Dimensions	86.36 x 45.72 x 53.34 cm; 3.44 kg
Use Object	ユニセックス(子供)

Gaiam (ガイアム) 子供用バランスボールチェア - クラシックな子供用安定性ボールチェア、オルタナティブスクールクラスルームの柔軟なデスクシート アクティブな学生向け満足保証付き ブルー/グリーン

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Gaiam (ガイアム) 子供用バランスボールチェア - クラシックな子供用安定性ボールチェア、オルタナティブスクールクラスルームの柔軟なデスクシートアクティブな学生向け満足保証付きブルー/グリーン