How to optimally level up to Apprentice with every class

Blacksmith: You can get a lot of XP by continuously smelting logs for charcoal with a large array of furnaces. What you should NOT be doing is crafting chests full of wooden tools for XP. You get 6 XP per smelted log, and only 3 XP per log turned into tools. Making tools with bamboo is still worth it though.

Builder: As of writing this, you can repeatedly break and replace flowers and other instabreak blocks for infinite and fast XP. In case this is patched out, you can still break and replace wool and stone bricks at a decent speed.

Farmer: Breaking short grass will give you 1 XP, and cooking food in a furnace gives varying amounts of XP depending on the food.

Guardsman: The fastest way of getting XP is by killing hostile mobs, but this can easily go wrong. If you go underwater with doors you can hit most mobs safely, but drowned can still get you. Passive mobs also give 1 XP per kill. If you have a mob farm, please do use that for leveling.

Healer: The only way to get healer is by crafting paper, and it takes a lot. 174 sugarcane to be exact. You will need to create a large sugarcane farm if you want a healer, and harvesting the sugarcane will require a decently leveled farmer.

Librarian: You will need to craft a lot of books to level up to librarian, and books require leather and paper. You should wait with getting this class before you get a cow farm and a sugarcane farm. Once you get Apprentice, you can level up faster by placing, breaking, and recrafting bookshelves.

Miner: Try to mine for coal and stone. If you find any fully grown amethyst crystals, mine those as well.