I mean unless you were good friends with her in high school...she's gonna know something is up just because you texted her out of the blue after like 3 years of not talking.
I mean think of it this way: if a girl you hardly talked to in high school randomly texts you after 3 years...you would think that was kind of strange wouldnt you? and you'd assume she was attracted to you and wanted to meet up.
This is not necessarily a bad thing. If she was attracted to you in high school or grew attracted to you since then (from hearing about you), then it really doesnt matter what you say...she is going to respond to any thing you send her, provided it's not super creepy or awkward.
If she wasnt/isnt interested in you, she'll either ignore you straight up...or she'll give you a generic polite response and probably ignore you soon after, or send one word replies until you stop texting.
Don't stress so much about your opener. I would probably use some callback humor to an inside joke or weird teacher you guys had. Something like "Okay this is completely random....but you'll never believe who I just saw at the bar!" She goes "blah, blah, who?", you go "That creepy old Mr.Smith we had for english". Not obviously this will only work if you guys were buddy buddy in that class and had a lot of good laughs...not if she was just some girl that sat two rows away that you thought was hot or never talked to.
Honestly dude....if you're just looking to get laid or date the hot girls you could never get in high school (which is a valid desire, dont get me wrong), hitting them up out of the blue is not the best way to go about it. If the girls you're talking about hitting up liked you in high school and you hung out with them at school or outside of school, then by all means, go for it.
Just be aware that randomly hitting up a girl you barely talked to can easily come off as creepy and random, UNLESS you know that she is/was attracted to you AND preferably, you guys were buddy-buddy way back when.
A safer bet would be to call up a whole group of friends that you're cool with (and that she has mutual friends with) and invite them to the bar for a mini-reunion of sorts. Once there, you ease into it, talk to her and ramp up your game.