Based on what we have see in Suramar, Thalyssra probably has a focus on one of the following schools of magic.
Transmutation spells change the properties of a creature, object, or environment. They might turn an enemy into a harmless creature, bolster the strength of an ally, make an object move at the caster’s command, or enhance a creature’s innate healing abilities to rapidly recover from injury.
Considering chronomancy falls under the Transmutation school, I think Elisande had a focus in this school.
Abjuration spells are protective in nature, though some of them have aggressive uses. They create magical barriers, negate harmful effects, harm trespassers, or banish creatures to other planes of existence.
We see Thalyssra use a lot of defensive spells, but rarely offensive ones. Which might be telling that she is an abjurer.
Enchantment spells affect the minds of others, influencing or controlling their behavior. Such spells can make enemies see the caster as a friend, force creatures to take a course of action, or even control another creature like a puppet.
Considering her research with the Withered, which is essentially just finding some weak willed creatures to more easily mind control, this could be telling of an enchantment background.
I think it is more than possible, considering her age, that she has some familiarity, or at least a working knowledge of all schools of magic. However, I think I would believe Enchantment to be her main focus.
We have not seen her do much in the way of Chronomancy, despite the nature of the Nightwell to be related to time. Considering I am already substituting the undefined nature of magic in WoW with D&D magic already, I think we can assume that Thalyssra and Nightborne in general, owe their magical prowness to the Transumtative nature of the Nightwell itself, but that doesn’t necessarily mean the school of magic a Nightborne uses is Transmutation.
It is possible that she is an Abjurer, however, Abjuration is the main school of magic a Spell Breaker would focus is. Within this school you have Counterspell, Sheild, Spell Steal, and mage armor. It’s very unlikely that an ageless abjurer would be afraid of a human mage, even if that mage was exceedingly powerful like Jaina.
In addition to that point, Thalyssra main contribution in Legion was her Withered Army, which leaves me to believe that her main school of focus is Enchantment, which would make sense. Thalyssra could be immensely powerful, but in the face of a strong willed creature, like Jaina, enchantment spells are next to useless. So it is not so much that Jaina out classes Thalyssra as a wizard, but rather Thalyssra is out of her element when it comes to spell duels.
