a) is, as you say, substandard, because it suggests you (or at least, your friends) are all still at high school together, which (from context) you / they are very likely not.
b) is neat and compact, and says what you mean. The implied context is sufficient to make the reader understand what you mean.
c) does not fit right with me. I don't know whether it is strictly ungrammatical, but it sounds clumsy and feels like it is in desperate need of the preposition "from" as in d).
d) is completely grammatically correct, and is completely explicit in that it does not require the context to be understood. However, in my opinion it is inferior to b) from a purely stylistic point of view, because it is more wordy than it needs to be.
Summary: b) and d) are correct, but b) is better stylistically.
EDIT: It has just occurred to me that b) and d) actually mean very slightly different things.
b) suggests that you were all at high school together.
d) (because it is "when I was in high school") suggests that the friends may not actually be at high school themselves. The speaker may have had older friends who had left high school by that time.
Hence it is not possible to determine exactly what the speaker means without further contextual clues.