pasuk 28, Rashi comments:
and they will go up from the Nile.A very quick and surface "What Is Bothering Rashi?" That is, why does Rashi feel the need to clarify "from the Nile?" The answer: otherwise, we would wonder what ועלו goes on. וְעָלוּ וּבָאוּ בְּבֵיתֶךָ is the phrase, so it could have meant "and they will alu in your house, and they will bau in your house." However, עלו means "coming up," so it is coming up from somewhere, and we know they came up from the Nile. Indeed, the pasuk begins וְשָׁרַץ הַיְאֹר, צְפַרְדְּעִים, וְעָלוּ וּבָאוּ בְּבֵיתֶךָ, and thus the Nile is explicitly mentioned, so they are coming from the Nile, from where they swarmed, and then entered into the house.
into your house and afterwards, into the house of your servants. He [Pharaoh] introduced the plan first, [as it is written:] “He said to his people…” (Exod. 1:9), and with him the retribution started. — [from Sotah 11a]
Indeed, one can point out that the trup is somewhat in agreement with this parsing. Here is the trup chart I generated for this pasuk: (I'm not going into the mechanics of how trup generates this tree. See William Wickes' book for detail.)
וְשָׁרַ֣ץ הַיְאֹר֮ צְפַרְדְּעִים֒ וְעָלוּ֙ וּבָ֣אוּ בְּבֵיתֶ֔ךָ וּבַֽחֲדַ֥ר מִשְׁכָּֽבְךָ֖ וְעַל־מִטָּתֶ֑ךָ וּבְבֵ֤ית עֲבָדֶ֨יךָ֙ וּבְעַמֶּ֔ךָ וּבְתַנּוּרֶ֖יךָ וּבְמִשְׁאֲרוֹתֶֽיךָ
Note that veAlu is separated from uVa`u beVeitecha by the disjunctive trup pashta. Of course, this might be entirely expected even without this, since a clause with three words in it needs to be subdivided, and perhaps this is a place we would expect such subdivision. I'll think about this point some more (and recheck Wickes about clauses beginning with verbs, and where the syntactic break should be).
Update: Thanks to Mississipi Fred MacDowell of On the Main Line who pointed out that Wickes is online (a good read, check it out). This is from page 56. Note that we should indeed usually expect the verbs to be together, and the fact that it is not appears to bolster Rashi's reading, at least from a trup perspective.
What would be the alternative? Well, עול is fairly common in Aramaic to mean "enter," and עלל is indeed a Biblical Hebrew word meaning "enter," (see e.g. Jastrow on this point) so it could mean that they will come to and enter the Egyptians' houses.
Even if it means ascend, picture a swarm of frogs swarming all over the people and their houses, covering them, and then getting into the houses. This could be the meaning of ועלו, to ascend upon, rather than to ascend from.
Indeed, perhaps the next pasuk could reinforce this:
|כט וּבְכָה וּבְעַמְּךָ, וּבְכָל-עֲבָדֶיךָ--יַעֲלוּ, הַצְפַרְדְּעִים.||29 And the frogs shall come up both upon thee, and upon thy people, and upon all thy servants.'|
And the next perek begins:
Note also how Shadal uses the word in his commentary to pasuk 29:
ובכה ובעמך : יעלו גם על בני אדם, כלומר במלבושיהם ובחיקם.