The small っ works like glue to the syllables around it and is not pronounced. If it were a normal, big つ it would be "gatsukou", but with a small っ, the "tsu" sound disappears and instead tells you to carry the first letter of the syllable to the right of it to the end of the syllable to the left of it - "gak kou". So, やった is "yat ta" not "yatsuta".
If a kana is small, it generally means that it shouldn't be counted as an individual syllable, but rather a part of the previous. I.e. ち = chi, よ = yo --> ちょ = cho き = ki, や = ya --> きゃ = kya where "kya" and "cho" are each one syllable.
(Please correct me if I'm wrong, I haven't been learning for a long time.)
This is probably a little late, but most IMEs will make っ by typing "xtu." Generally, typing an "x" before anything you want to make small and standalone works. So I can make ゃ without having き or something else with it. Though, honestly, just typing the word as normally romanized with the double consonants will automatically add the っ (gakkou = がっこう).
My SwiftKey keyboard on the phone, there's a special button to go through that letter and it's different pronunciations. っつづ
"gakkou" Remember that Japanese is split up into these syllable-like things called mora, which all take up the same amount of time, regardless of position. In other words, there is no stressing syllables in Japanese, so every component (mora) is pronounced for the exact same length of time.
So, がっこう "gakkou" is made up of 4 mora: ga - k - ko - u.
The が "ga" is just as it seems. Say this for, say, 0.2 seconds.
The っ extends the following consonant. So, since that would be "k" (in こ "ko") your mouth gets frozen in a "k" position for another 0.2 seconds. (Hence the double "k" romanization)
The こう "kou" at the end of the word is called a long vowel. Because the "o" and "u" sounds are paired up, the う "u" serves to extend the "o" sound in こ "ko", which gets pronounced for double the amount of time (0.4 seconds).
Keep in mind that 0.2 seconds was just an example.
The mini-tsu signifies something like a glottal stop, like you're holding on to the previous syllable before saying the next, or holding onto the consonant at the start of the next character before saying it fully. This stoppage is symbolised using english letters by writing a double consonant. The "u" on the end elongates the last vowel. So in essence, when you see a doubled vowel or consonant in the english letters, it means getting stuck on that letter – when it's a vowel you're saying it longer, and when it's a consonant you get it stuck in your throat.
がこ = gako
がっこう = gakkou / gakko– / gakkō (depending on how you prefer to write it)