Shikotsumyaku Armor

Name: Shikotsumyaku Armor
Rank: A
Type: Taijutsu
Element: N/A
Range: Self
Description: The main capabilities of Shikotsumyaku is allowing the user to manipulate the speed of their bones' growth as well as the location of calcium deposits. By infusing their calcium with chakra, Kaguya are able to manipulate the growth and properties of their bones to form a unique skeletal structure. By doing this, the user is capable of generating an armor of dense bone underneath their skin. This additional layer of protection grants the user an impenetrable skeletal frame, which can resist almost any type of physical attack. Kaguya bones can even withstand a chakra-infused blade, which is usually enough to cut through anything. The user of this technique gains the additional 50 Durability of their bone structure placed directly underneath their skin. In addition, constant chakra must be paid an upkeep price for each post this skeletal form is maintained.

I'm not a mod, but this technique in no way needs to be S-Rank. It's very basic to the Kaguya; (it would seem anyway, since they can remove their spine with an A-Rank, and Kimimaro did this with almost no preparation) you could probably drop it to B-Rank. (Though the reason I am posting this is I'll probably learn this technique as soon as I get the EXP for it.)

I could be wrong...and I probably am

But considering it uses chakra infused bones...from a clan...wouldn't it be a Ninjutsu technique?

Not at all. All Kaguya clan Taijutsu involve infusing chakra into their bones. The only Shikotsumyaku technique that is considered a ninjutsu is the Dance of the Seedling Fern because it is an external ability which sprouts bones from the environment itself. The Naruto Wiki states "Out of all the techniques shown through the Shikotsumyaku abilities, this is the only one that is ninjutsu, rather than a taijutsu." This armor is just a defensive method of infusing the bones with chakra for a stronger guard against attacks, rather than the offensive use of sprouting them out as weapons. Plus, Kimimaro actually performed this ability in order to prevent himself from being crushed by Gaara's sand coffin:

Demonstration:

Approved.

A-rank upkeep cost.