I would only get one unless they are already bonded in a pair or trio, bonding even two can take a long time and requires a lot of patience and may still fail. Also keep in mind with more then one that you will need to have or the ability to create (like divide a cage) as many cages as you have chins in case things don't work out. Although a lot of chins like company there are some that don't and it's always a chance that even after years together they will decide they want to be anymore and you need to house them separate. So really it comes down to do you have room for one cage? or more? For example I currently have 2 chins and only have room for one cage, but I have a double Ferret Nation which can easily be turned into two cages if needed without taking up extra room. Also you will need one of everything per chin, so for two you'll want 2 food bowls, 2 water bottles, 2 hay holders, etc to prevent a possible fight over resources. Also, if needing to be separated later, you will need to do separate play times.
You being in school all day isn't an issue for chins, they sleep most of the day. They are awake most in the early morning, sleep most of the day, and are up again in the evening, pretty much up during the cool parts of the day, sleeping during the hottest and coldest parts.
As far as age I would say anything over a year, so from the options 1-15yrs it doesn't really matter. Over 1 year and they have generally passed puberty so you know better what you have in terms of personality. However if it was me I'd choose a younger one (under 5) as my first chin so I'd have more time with it, chins live 15-20 years so a 15 year old you'd have limited time with. Chins are crazy as kits, then most calm at least a little once they hit puberty but really don't slow down more after that, unless they are sick, until they die. There are some chins that run around, wall surfing, and popcorning like a crazy chin, you'd never know they are old, right up till the day they die.
Another thing to consider is single chins tend to bond more closely to their owner since they are their only socialization. But at the same time, you are their only socialization so you need to make it a top priority to give them attention everyday. They have similar social requirement to a parrot, you can't ignore them or they will get lonely and depressed. I have had multiple chins in the same room, not caged together but near enough so they can "talk" to, see, and smell other chins. So if you do decide on more then one even if they don't work out living together they can still socialize. Some also enjoy play mates but not cage mates, so they can still play together just not share a cage.
