This is my attempt to properly balance the ridiculously broken silvery barbs as released in Strixhaven: A Curriculum of Chaos.

The spell no longer imposes super-disadvantage (only regular disadvantage) and only on an attack roll or ability check; this means that you can no longer force a target to re-roll a successful save that you triggered during your own turn (e.g- dominate monster then silvery barbs as a reaction if the target succeeds on the save), it also means you can no longer trivially bypass legendary resistance using a 1st-level spell.

While imposing disadvantage in advance makes this more preemptive/speculative, it is still somewhat as strong as shield (disadvantage is equivalent to -5 to hit, so similar to having +5 AC) except that this is trading the ability to wait to be hit, and lasting until your next turn, for the freedom to also affect ability checks and be able to help allies. So it's like a lesser, yet more flexible Shield that you can also use for support and brute forcing checks.

As an extra sweetener since it's a bit more risky than shield, the spell retains the ability to also grant advantage; if the attack/check you targeted is failed, you now impose guiding bolt like "next attack or check" advantage that you or an ally can then exploit, to represent the enemy being distracted/surprised.

The spell should remain a solid defensive/protective spells that's also useful for all opposed skill checks (including Grapple, Shove etc.) and any other attempts to interfere with the checks of another while outside of combat. It can also function as a possible counterspell breaker if your DM runs enemies that have counterspell, as you can force them to re-roll a successful counterspell check rather than having to take counterspell yourself. Or you could use it to force an enemy to roll initiative with disadvantage to put them down in the initiative order. So it remains a very, very flexible 1st-level spell.

Overall I feel that this is much, much better balanced than the official spell which would be overpowered even if it were 2nd-level like the other signature spells, but as a 1st-level spell is utterly broken to the point it never should have been released as-is.

I went a different route for my own revision. 

You magically distract the triggering creature and turn its momentary uncertainty into encouragement for another creature. The triggering creature must reroll the d20 and use the lower roll. A creature immune to the Charmed condition can make a Charisma saving throw to avoid the effect, ending the spell on a success. 

(This suddenly gives over 500 5e creatures a possible saving throw). 

Also, I revised the reaction trigger:

"...which you take when another creature you can see within 60 feet of yourself succeeds on a perceivable attack roll, ability check, or saving throw."

Player's Handbook, chapter 10 : "Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature’s thoughts, typically goes unnoticed, unless a spell says otherwise."

Thus, for example, you can affect a Dexterity saving throw made to escape the full brunt of a fireball, or an Strength (Athletics) check made to jump over an obstacle, but not a save made to resist a mind spike spell intrusion, or an Intelligence (Religion) check that a creature is making for some reason.

If there's any doubt whether a particular attack roll, ability check, or saving throw can be perceived, the DM adjudicates.

At the very least, there is no doubt that the spell needed balancing.