Show the verse from Scripture that a high priest must be from Aaron.
Well, Chapter 8 in Leviticus is the installation of Aaron and his sons for the Priesthood, altogether. However, the high priesthood was given to Aaron's grandson Pinchas into eternity in Numbers 25:13.
The Lord spoke to Moses, saying: י. וַיְדַבֵּר יְהֹוָה אֶל משֶׁה לֵּאמֹר:
11. Phinehas the son of Eleazar the son of Aaron the kohen has turned My anger away from the children of Israel by his zealously avenging Me among them, so that I did not destroy the children of Israel because of My zeal. יא. פִּינְחָס בֶּן אֶלְעָזָר בֶּן אַהֲרֹן הַכֹּהֵן הֵשִׁיב אֶת חֲמָתִי מֵעַל בְּנֵי יִשְׂרָאֵל בְּקַנְאוֹ אֶת קִנְאָתִי בְּתוֹכָם וְלֹא כִלִּיתִי אֶת בְּנֵי יִשְׂרָאֵל בְּקִנְאָתִי:
12. Therefore, say, "I hereby give him My covenant of peace. יב. לָכֵן אֱמֹר הִנְנִי נֹתֵן לוֹ אֶת בְּרִיתִי שָׁלוֹם:
13.
It shall be for him and for his descendants after him [as] an eternal covenant of kehunah, because he was zealous for his God and atoned for the children of Israel." יג. וְהָיְתָה לּוֹ וּלְזַרְעוֹ אַחֲרָיו בְּרִית כְּהֻנַּת עוֹלָם תַּחַת אֲשֶׁר קִנֵּא לֵאלֹהָיו וַיְכַפֵּר עַל בְּנֵי יִשְׂרָאֵל:
Jesus is the High Priest in accordance to the following,
Psalm 110:1,4 (NIV)
The LORD says to my lord: “Sit at my right hand until I make your enemies a footstool for your feet.”
The LORD has sworn and will not change his mind: “You are a priest forever, in the order of Melchizedek.”
Concerning that Psalm, it was written by a soldier about King David.
110:1 Regarding David, a psalm. The Lord to my master; "Wait at My right, until I make your enemies a footstool at your feet." א. לְדָוִד מִזְמוֹר נְאֻם יְהֹוָה | לַאדֹנִי שֵׁב לִימִינִי עַד אָשִׁית אֹיְבֶיךָ הֲדֹם לְרַגְלֶיךָ:
Vowelization counts, and the word Adonee (as it would be pronounced), which is Master, is not the same as Adonai, which is the way it would be pronounced if it were referring to God, as our Lord.
110:4 The Lord swore and will not repent; you are a priest forever by my word [you are a] Righteous King. ד. נִשְׁבַּע יְהֹוָה | וְלֹא יִנָּחֵם אַתָּה כֹהֵן לְעוֹלָם עַל דִּבְרָתִי מַלְכִּי צֶדֶק:
There is no "order of Malkizedek."
The praise that the soldier had written regarding King David called him a priest, not because he WAS a priest, but because his actions on the battlefield encouraged the soldiers around him to act morally and ethically.
Besides... If you are looking to this psalm for your justification connecting King David and Jesus, it is lost, as if King David isn't paternally connected to Jesus, the whole "forever" aspect is lost.